By strand sequence we mean the sequence of the strands in a given sheet, following the backbone of the protein from the N-terminus to the C-terminus. As an example, consider the following three sheet motifs:

The first motif, the standard "up-down-up-down" motif, has the strand sequence 1-2-3-4. The second motif, the "greek key", has the strand sequence 1-4-3-2. The third motif, for example found in protein L, has the strand sequence 2-1-4-3. To take the axes of symmetry into account, the first strand has to be on the left side of the sheet (for sheets with an odd number of strands we require the second strand to be on the left side if the first strand is the center one). And remember that you can not choose the same position twice in the form that you fill out - it will give you an error message on the output page!