:MARKer:X:SOURce:SPARameter
Instrument:
                                                N1000A
                                                DCA-M
                                            Meas. mode:
                                                Scope
                                                Eye
                                                TDR
                                            Flex Apps:
                                                FlexDCA
                                                FlexRT
                                            Command Syntax
:MARKer:X{1:4}:SOURce:SPARameter <parameter>
                                            Where the {1:4} specifier identifies one of four markers. For example, :MARKer2.
The <parameter> argument is an S-parameter name with an underscore (_) character between the numbers. For example, the argument for an S21 parameter is S2_1.
Query Syntax
:MARKer:X{1:4}:SOURce:SPARameter?
                                            Description
In TDR mode, use this command along with the :MARKER:X:SOURce:DUT command to select a tracking marker's source waveform. This command selects the TDR S-parameter.   This command applies only to tracking markers and does not affect manual line markers. 
Requires FlexDCA revision A.04.00 and above.