As is often pointed out, the Equation Editor will accept most VBA statements and operators in addition to the extremely limited set of statements and operators it lists in the Help file. Now I am surprised to notice that SW2015 is proactively switching out the "supported" statement "IF" for the unsupported VBA statement "IIF". weird!
I noticed this behavior, and as a test I made some simple, non-nested IF equation statements, and linked them as an external text file into the part file via the equation editor interface. Voila, upon import they changed to IIF. I rebuilt the model, saved it, closed it, reopened the linked text file outside of SW, yep, still just IF. I reopened the part file, equations still showing as IIF. I relinked the equation text file, still IIF. I unlinked the equations, edited them directly to be IF, not IIF. Close the line, *POP*, right back to IIF.
What's up with this? Has anyone else been experiencing this?