G21 (metric)
F100 (note slow feed)

G91 (relative mode)
G1 X10

; this sub switches to G20 (inch) and G90 but protects the caller by M70/M71
; it also switches to a much higher feed
O<g20sub> callsub

G1 X10 (this move still relative and in metric units, note slow feed restored)
m2