(1) Adding a number or incrementing a variable
set var1=1
set var2=1
set /a sum1=%var1%+%var2%
echo %sum1%
(2) extract a length of substring from an offset and
set var=one.twothree
set /a var2 = %var:~4.3%
echo %var2%
will yield "two" (3 characters from offset 4 i.e. 5th character)
set var=one.twothree
set /a var2 = %var:~4%
echo %var2%
will yield "twothree" (substring after offset 4 i.e. 5th character onwards)