TSRef - TypoScript Reference
3.1.2. Calculating values (+calc):
Sometimes a datatype is set to "something +calc". "+calc" indicates that the value is calculated with "+-/*". Be aware that the operators has no "weight". The calculation is done in the order of the operators.
Example:
45 + 34 * 2 = 158 (which is the same as this is ordinary arithmetic: (45+34)*2=158 )