https://linux-tc-notes.sourceforge.net/tc/doc/cls_u32.txt:

The base operation of the u32 filter is actually very simple. It extracts a bit field from a 32 bit word in the packet, and if it is equal to a value supplied by you it has a match. The 32 bit word must lie at a 32 bit boundary.

Funny enough I arrive at this the most when I play the Triassic and my opponent goes for a Cretaceous game structure.