WUFI

Pardon me if this is a dumb question, but what does the frame factor represent in the glazing definition? I thought it was the U-factor of the window frame, but then I got a message telling me that the value I used was invalid.

The frame factor is the ratio of transparent area of your window to the whole window.
The U-factor in WUFI Plus is the overall U-factor of the whole window (glazing and framing). So for your heat loss/gain the whole window area is multiplied with the U-factor.
Your solar gains on the other hand can only be achieved through the transparent area of the window element. A differentiation is therefore necessary.
A frame factor of 0.7 for a window of 1 m² means, that there are 0.7 m² transparent glazing area and 30 % of the window opening in the wall is not transparent.