Siemens SIMATIC S5 Manual page 223

Function
Assignment within a message (numbers 0 to 12) in data block DB-A
For changes in the data words DW n to DW n + 6, it is sufficient to transfer the changes with the
command AS.
Changes in data words DW n + 7 to DW n + 15 must be transferred with the command AE
(or KS/PA). Data words DWn to DW n + 6 are also transferred.
When changing the following:
– bit 2, control byte 1
– bits 3 to 7, control byte 1 in pure Pt100 operation or
– bits 1 and 3, control byte 2
the command AE (or KS/PA) should be used.
5–18
IP244
C79000–B8576–C861–02