When 32 bit inode numbers are used on a volume larger than 1TB in size, several changes occur.

A 100TB volume using 256 byte inodes mounted in the default inode32 mode has just one percent of its space available for allocating inodes.

XFS will reserve the first 1TB of disk space exclusively for inodes to ensure that the imbalance is no worse than this due to file data allocations.

It is no longer possible for file data to reside in the same AG as the parent directory's inode.

XFS will instead "rotor" through the upper AGs as it allocates space for files, putting each file in a new AG to evenly spread the I/O load.