If you use read/write blocks, then there is no need to select modbus mapping. M221 maps %M and %MW variables on behind automatically to modbus addresses.
Modbus mapping is for different configuration.
On master side you program write/read blocks according help files and block points with IDx number to partner PLC. (ID1 = slave 192.168.1.10).
You can have 16 different IDs on master side. (And slave IP addresses)
On slave side, you only need to give IP address and select enought %M and %MW variables to use.
(Default setting is that highest bit or word is highest uesd variable on program. On settings you can select that there is more variables, than you use on PLC program)