Why in their condition in sphere r fixed ,in disk thetha fixed ,in half disk phi fixed?????? why??

Surface Elements: 1. Spherical Surface r-fixed ds,= dl, dl, f=r sin0 de do f l R ds = r sin 0 de do r and ds, sin e de do l S S =R°I-cos )G[ol =R-cOS T + COs 0127T = R(2]x 27t 4tR 2. Disk Surface -fixed = 90