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AllowBurrowing
As it says on the tin; if yes allows unit with Subterranean Locomotor to dig in and out, if no they're not allowed to do that.
Default for this is true/yes.
Note this has no real effect in stock RA2 and YR since they don't have any units using the Subterranean Locomotor, but custom maps and custom game modes can make units use them. Mods too, naturally.
See also
Subterranean Locomotor