The index groups sheets by discipline: general, civil, architectural, structural, mechanical, plumbing, electrical and others.
Some indexes also show the issue date or revision of each sheet. That column tells you which sheets an addendum changed.
Check your set against the index before you start. A missing sheet means missing scope. A plan room download can drop sheets.
Use the index to find sheets outside your discipline that affect you. A food service contractor still needs the plumbing and electrical sheets.