Consider the following example with only two levels:
A
A1
B1
B
A2
B2
The default behaviour of the TreeGrid is to filter both levels. For example, a filter of "A" would give us this:
A
A1
If we constrained filtering to the second level, a filter of "A" would give us this:
A
A1
B
A2
If we constrained filtering to the first level, a filter of "A" would give us this:
A
A1
B1