These numbers are specified when accessing the buffer memory of the intelligent function module (bus conversion module). 
The number assignment method differs depending on the MELSEC series as follows:

For I/O number (start XY)
Figure 1: MELSEC iQ-R setting example (GX Works3)

• Number assignment method: The numbers (I/O numbers and module number) are automatically assigned to the I/O module and intelligent function module in order from the closest to the CPU module. 
The assigned numbers can be changed. 
• Assigned number: Starting from 0 in hexadecimal. 
The assignable numbers for one module ranges from 0 to 16, 16 to 48, or other specified ranges. 
This ranges represent the number of I/O points each module has. (Figure 1 (a))

For module number (module No.)
Figure 2: MELSEC iQ-F setting example (GX Works3)

• Number assignment method: The numbers (I/O numbers and module number) are automatically assigned to the intelligent function module and bus conversion module in order from the closest to the CPU module. 
Numbers are not assigned to the I/O module. 
The assigned numbers cannot be changed. 
• Assigned number: Starting from 0 or 1 in decimal. 
A single number is assigned to one module. (Figure 2 (b))