The BAT area follows the free list. The size and number of BATs is defined when the filesystem is formatted. BATs are arrays of block addresses for the blocks that contain the data of the file. The number of BATs gives a hard upper limit to the number of files permitted. Usually this is set to equal the number of directory entries. There is little point in making it larger, but it may be useful to set it smaller if the minimal size of the directory exceeds the desired maximum number of files.
The size of each BAT represents a hard upper limit on the size of a file. BAT size should be set to cover the expected range of file sizes. Larger data sets can be handled at application level by splitting the data across several files.