Suppose you are comparing paper and size options and see the fold you want is no longer available. In that case, the fold is not compatible with either the size or the paper that you have chosen.

For example, a Quarter Fold is possible on a 10pt or 12pt card stock, but not on 14pt. Quarter Fold requires the paper to be folded until there are 4 layers at the central fold, and the 14pt is too thick. 

Another example would be the Double Gate Fold, which is not available for 11"x8.5" or 9"x12" but is available for 8.5"x14" and up. The 4 panels on a Double Gate Fold are not the same size. There needs to be sufficient length for the two outer panels to be shorter than the inside panels, but still large enough for a reasonable amount of content.