load.txt

Preindexed:
The offset modification is performed *before* the
base is used as the transfer address.

The W bit gives optional auto increment and decrement addressing modes.
The modified base value may be written back into the base (W=1), or the old base value may be kept (W=0).

So, alu result will be used for writing/reading. and This W bit says if this alu result is written to base register or not.

Postindexed:
The offset modification is performed *after* the
base is used as the transfer address.

In the case of post-indexed addressing, the write back bit is
redundant. Therefore post-indexed data transfers always write back the modified base. 

So, alu result will not be used for writing/reading, rather out_data from base register will be used.
No matter what W bit is, we write alu_result to base register.