Siemens Simatic s7-1500 Manual page 17

The figure below shows the assignment of the address space with submodules 3 and 4.
Figure 4-4
Reference
You can find information on the module-internal shared input/shared output (MSI/MSO)
function in the section Module-internal shared input/shared output (MSI/MSO) of the function
manual PROFINET with STEP 7 V13
(https://support.industry.siemens.com/cs/ww/en/view/49948856).
DI 16x230VAC BA Digital Input Module (6ES7521-1FH00-0AA0)
Manual, 09/2016, A5E03485975-AD
Address space for configuration as 1 x 16-channel DI 16x230VAC BA MSI
Address space
4.1 Address space
17