When does someone use rgmii-txid and rgmii-rxid?

When there is an extra long RX clock line on the PCB, but not the TX
clock line, you would use rgmii-txid. If there is an extra long TX
clock line, but not RX clock, you would use rgmii-rxid. You do not see
this very often, but it does exist:

arch/arm/boot/dts/nxp/ls/ls1021a-tsn.dts

/* RGMII delays added via PCB traces */
&enet2 {
        phy-mode = "rgmii";
        status = "okay";

	Andrew