The black diamonds on the top scale of a tape measure are for truss layouts for eight foot sheet goods. They start at 19.2 inches, which was originally used in metric layouts. 96 inches divided by five gives 19.2 inches, so five trusses per sheet.
There are also red stud markings every 16 inches on a tape measure, allowing for a standard layout of six studs per 8-foot section. The small black diamonds are sometimes represented by small triangles, although diamonds are more common. These markings speed up the measuring process and provide a quick visual guide for the tape measure user.