For the general description of PTRS, I would just quote the explanation from VI-B of this paper  since it is better described than I would have done.

PTRS is used for tracking the phase of the local oscillator at the receiver and transmitter. This enables suppression of phase noise and common phase error, particularly important at high carrier frequencies such as millimeter wave. Due to the properties of phase noise, PTRS can have low density in the frequency domain but high density in the time domain. PTRS can be present both in the downlink (associated with PDSCH) and in the uplink (associated with PUSCH),. If transmitted, PTRS is always associated with one DMRS port and is confined to the scheduled bandwidth and duration of PDSCH/PUSCH,. Time and frequency densities of PTRS are adapted to signal-to-noise-ratio (SNR) and scheduling bandwidth.

Time Domain Symbol Location

Time domain symbol location of PDSCH PTRS is specified by 38.211 7.4.1.2.2 and following is my interpretation.

Frequency Domain RB and Subcarrier Location of PDSCH PTRS

Frequency domain symbol location of PDSCH PTRS is specified by 38.211 7.4.1.2.2 and following is my interpretation

RRC Parameters for CSI RS

PDSCH-Config ::=                        SEQUENCE {

    dataScramblingIdentityPDSCH             INTEGER (0..1007)    OPTIONAL,

    dmrs-DownlinkForPDSCH-MappingTypeA      SetupRelease { DMRS-DownlinkConfig }  OPTIONAL,   

    dmrs-DownlinkForPDSCH-MappingTypeB      SetupRelease { DMRS-DownlinkConfig }  OPTIONAL,  

    tci-StatesToAddModList                  SEQUENCE (SIZE(1..maxNrofTCI-States))

                                                OF TCI-State    OPTIONAL,   -- Need N

    tci-StatesToReleaseList                 SEQUENCE (SIZE(1..maxNrofTCI-States))

                                                OF TCI-StateId  OPTIONAL,   -- Need N

    vrb-ToPRB-Interleaver                   ENUMERATED {n2, n4},

    resourceAllocation                      ENUMERATED { resourceAllocationType0,

                                                         resourceAllocationType1,

                                                         dynamicSwitch},

    pdsch-AllocationList                    SEQUENCE (SIZE(1..maxNrofDL-Allocations))

                                                OF PDSCH-TimeDomainResourceAllocation ,

    pdsch-AggregationFactor                 ENUMERATED { n2, n4, n8 } OPTIONAL, 

    rateMatchPatternToAddModList            SEQUENCE (SIZE (1..maxNrofRateMatchPatterns))

                                                 OF RateMatchPattern    OPTIONAL, -- Need N

    rateMatchPatternToReleaseList           SEQUENCE (SIZE (1..maxNrofRateMatchPatterns))

                                                 OF RateMatchPatternId     OPTIONAL, -- Need N

    rateMatchPatternGroup1                  SEQUENCE (SIZE (1..maxNrofRateMatchPatterns))

                                                 OF RateMatchPatternId     OPTIONAL, -- Need R

    rateMatchPatternGroup2                  SEQUENCE (SIZE (1..maxNrofRateMatchPatterns))

                                                 OF RateMatchPatternId     OPTIONAL, -- Need R

    rbg-Size                                ENUMERATED {config1, config2},

    mcs-Table                               ENUMERATED {qam64, qam256},

    maxNrofCodeWordsScheduledByDCI          ENUMERATED {n1, n2} OPTIONAL,   -- Need R

    prb-BundlingType                    CHOICE {

        static                                  SEQUENCE {

            bundleSize                              ENUMERATED { n4, wideband }  OPTIONAL  

        },

        dynamic                                 SEQUENCE {

            bundleSizeSet1                          ENUMERATED { n4,

                                                                 wideband,

                                                                 n2-wideband,

                                                                 n4-wideband

                                                                }  OPTIONAL,   -- Need S

            bundleSizeSet2                          ENUMERATED { n4,

                                                                 wideband

                                                                } OPTIONAL    -- Need S

        }

    },

    zp-CSI-RS-ResourceToAddModList          SEQUENCE (SIZE (1..maxNrofZP-CSI-RS-Resources))

                                                OF ZP-CSI-RS-Resource   OPTIONAL,   -- Need N

    zp-CSI-RS-ResourceToReleaseList         SEQUENCE (SIZE (1..maxNrofZP-CSI-RS-Resources))

                                                OF ZP-CSI-RS-ResourceId OPTIONAL,   -- Need M

    aperiodic-ZP-CSI-RS-ResourceSetsToAddModList    SEQUENCE (SIZE (1..maxNrofZP-CSI-RS-Sets))

                                                OF ZP-CSI-RS-ResourceSet   OPTIONAL, -- Need N

    aperiodic-ZP-CSI-RS-ResourceSetsToReleaseList   SEQUENCE (SIZE (1..maxNrofZP-CSI-RS-Sets))

                                                OF ZP-CSI-RS-ResourceSetId   OPTIONAL, -- Need N

    sp-ZP-CSI-RS-ResourceSetsToAddModList   SEQUENCE (SIZE (1..maxNrofZP-CSI-RS-Sets))

                                                OF ZP-CSI-RS-ResourceSet     OPTIONAL, -- Need N

    sp-ZP-CSI-RS-ResourceSetsToReleaseList  SEQUENCE (SIZE (1..maxNrofZP-CSI-RS-Sets))

                                                OF ZP-CSI-RS-ResourceSetId   OPTIONAL, -- Need N

    ...

}

DMRS-DownlinkConfig ::= SEQUENCE {

    dmrs-Type                                ENUMERATED {type2} OPTIONAL,

    dmrs-AdditionalPosition                  ENUMERATED {pos0, pos1, pos3} OPTIONAL,

    maxLength                                ENUMERATED {len2} OPTIONAL,

    scramblingID0                            INTEGER (0..65535) OPTIONAL,

    scramblingID1                            INTEGER (0..65535) OPTIONAL,

    phaseTrackingRS                          SetupRelease { PTRS-DownlinkConfig } OPTIONAL,

    ...

}

PTRS-DownlinkConfig ::= SEQUENCE {

    frequencyDensity                          SEQUENCE (SIZE (2)) OF INTEGER (1..276) OPTIONAL,

    timeDensity                               SEQUENCE (SIZE (3)) OF INTEGER (0..29) OPTIONAL,

    epre-Ratio                                INTEGER (0..3) OPTIONAL, -- Need S

    resourceElementOffset                     ENUMERATED { offset01, offset10, offset11 }OPTIONAL,

    ...

}

frequencyDensity : Presence and frequency density of DL PT-RS as a function of Scheduled BW. If the field is absent, the UE uses K_PT-RS = 2.

timeDensity : Presence and time density of DL PT-RS as a function of MCS. The value 29 is only applicable for MCS Table 5.1.3.1-1. If the field is absent, the UE uses L_PT-RS = 1.

resourceElementOffset : Indicates the subcarrier offset for DL PTRS. If the field is absent, the UE applies the value offset00. See 38.211-6.4.1.2.2.1 for the detailed usage of this parameter.

Reference