The context menu command Insert condition... opens the window Condition properties. In this window you can combine the variables (length, diameter, etc.) of two parts with operands =, <, <=, >, >=, <>.

The syntax for the first part of the condition (condition 1) from the Variable browser (see further below) is solved as follows:

PL@D means value of variable "D" of the part plate "PL".

The entire expression PL@D = FS@DG in the Condition properties window is:

The value of the variable "D" of the plate "PL" corresponds to the value of variable "DG" of the the butterfly screw "FS".