G98 is initial plane return and G99 is referance plane return.
G0 G90 X1 Y-1. M3 S2500
G43 Z2. H1 M8
G81 G98 Z-.25 R.1 F10.
In G98 the tool will rapid to Z.1("R" plane), drill the hole rapid back to Z2.("Initial" plane) move to next position, rapid to Z.1 drill Rapid back to Z2. etc In G99 mode at 1st hole Z would rapid to Z.1("R" plane) ,drill hole, rapid back to Z.1("R" plane) move to next hole, drill etc.
I use G98 as a safety to clear clamps and fixtures
Hope this is clear