These are different variants that can be used on plain tiles (AAAA) to add more variety.
They should have all the same properties (passability, etc).
Variants are per surface. Meaning that each surface has its own variants. That is why we have a separate folder for them.

name_#
name - Surface name

# - Variant number:
 0 - is the default tile (in case we want to override it, e.g. snPavement)
 1..n - variants (max variants count is hardcoded in MAX_TILE_PLAIN_VARIANTS)
