When GAM returns no ad (or fails) and SampleFallback is on, the break is filled with a sample. Random picks one entry uniformly at random from the pool of its vpos below; other applies to any vpos without its own column, unchecking everything for a position disables the fallback there. Position-aware ignores the pools and always asks for Google's position-aware sample, which GAM answers with the creative made for that vpos (distinct pre-roll, mid-roll and post-roll videos). The other catalog entries return the same video whatever the position.
sample
parameters
preroll
midroll
postroll
other
test
Custom VAST tagsplaceholders {vpos} and {correlator}; a missing vpos/correlator parameter is appended