Mitsubishi Electric MELSEC Q Series Programming Manual page 27

Internal variables
A function does not use internal variables. It uses devices assigned directly to each input variable and repeats operations.
A program that outputs the total of three input variables
Ex.
When using a function (FUN1)
Function
X0
D109 D109
D110
D110
D111 D111
A function block uses internal variables. Different devices are assigned to the internal variables for each instance of function
blocks.
Ex.
When using function blocks
Instance A
Function block
D10 D6200
D11 D6201
D12 D6202
FUN1 D120
D10
D6203 D13
D11
D12
Instance B
Function block
D6210
D6213
D6211
D6212
4 PROGRAM CONFIGURATION
D13
25
4.2 POUs
4