Unilateral means 'one-sided' while bilateral means 'two-sided'. For unilateral profile of a surface, the print MUST indicate on which side of design the surface variation is allowed to be on. And, it means that all the tolerance indicated in the call out is allowed in that 'direction' from design, while none is allowed in the other direction. This makes a 'perfect' part right on the tolerance limit, not a good way to design. Bilateral (the normal, intelligent way to tolerance) means that exactly 1/2 of the tolerance shown in the call out is applied in EACH direction from design for a total + and - tolerance equal to the call out.