The door rule #208 searches only slabs and stairs under the doors (not coverings) at the moment.
The distance must be less than 1/10 of the door height, otherwise there will be this “missing slab” result.
The slab or stair under the door must horizontally overlap at least a bit, otherwise the slab is not found.
I believe this result type is added for bringing up the most obvious cases that might harm the results of the other checks in this rule.
The distance rule #222 would be better for checking the distance of door bottom level and floor.
Note:
The max threshold parameter in the table is for different check. This check is making sure that the door accessible and not having too high threshold (Schwellehöhe). The door threshold is taken from property sets usually.