It's the separator that excel uses between the worksheet name (Tabl.1&3 Cohorts)
and the address of the cell (C70).

It divides the sheet name from the cell reference.

A 3-D cell reference includes the worksheet name, an exclamation point (!),
and the cell or cells referenced. The exclamation point is just the symbol
used as a seperator between the worksheet name and the cell address.

It identifies the text between the = sign and the ! as the sheet name.

Or, the sheet name identifier.

Biff