what does the cell in series and active area mean in the PhotovoltaicPerformance:EquivalentOne-Diode?

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Note: I'm answering purely from the E+ standpoint, as I can't comment on Design Builder specific things since I don't use it.

Generator:Photovoltaic is the one that specifies which PhotovoltaicPerformance:XXX object it uses (XXX can be one of three types: Simple, Sandia, or EquivalentOne-Diode), which constitutes a single Module and the arrangement of modules in series and parallel via the Number of Modules in Series and Number of Series Strings in Parallel fields, cf I/O.

The Performance:EquivalentOne-Diode object has a field Number of Cells in Series, which isn't the same... This one is used to determine the voltage. If you want to have a lot of fun, please be my guest and read the Engineering Reference for the One-Diode model (the TRNSYS model) here. The parameter in question is $N_s$, which is referenced only at the end of the "PV Section 1: Four-Parameter Model".

$$A = \frac{\gamma }{{{N_s}}}$$

It is not super clear and I can't match it to the source code (though that's perhaps because I don't want to spend a few hours on this answer...). Anyways, in this case I think the source code is more readable: basically the number of cells in series $N_s$ (in source code that's CellsInSeries) gets multiplied by the Semiconductor Bandgap value. cf Photovoltaics::CalcTRNSYSPV