Left and Right are "Sides"; Front and Rear are "Ends"; Top and Bottom are...?

Solution 1:

Planes, the top plane and bottom plane. Technically all the sides of a cube are planes.

Solution 2:

You want words such as "sides" and "ends" to refer to absolute directions. But your readers might not understand them like that. Often, "sides" refers to the largest-area pair of opposite surfaces, and "ends" to the smallest-area pair. For example, if something is long left to right, but narrow and thin, you might talk of its left end and right end.

In your situation if the back surface is so long compared to its width that there's no danger of the user mistaking one sort of flange for the other sort, you could describe the longer two as side flanges and the shorter two as end flanges. Not because "end" specifically means top and bottom, but because "sides" are longer than "ends".

But to judge from your diagram the back surface looks almost square to me, and I fear that a user might think of the member as having four sides/edges and needing four side flanges. I'd favour being clear even if this means using more words. For example give instructions for the bottom horizontal flange, and then say attach the top horizontal flange similarly.