For some features, such as "Extruded Boss/Base", you are able to use equations as input values to its various fields (Figure 1). Then, for some features, such as "Flex" you can't use equations as inputs (Figure 2).
Which features can or can't do this seem just haphazard. Is it really just that simple, that some just randomly don't let you do it? is there a hidden setting I'm missing? there's no way to trick SolidWorks into allowing this?
Using Windows, SolidWorks 2017, service pack 2.0
Figure 1, successfully using a equation as the input to a feature.
Figure 2, unsuccessfully using a equation as the input to a feature.