The sub tag accepts two parameters and returns as its output the value obtained after subtracting the second parameter from the first.
The parameters may be literal strings, variables or output of other tags.
<cms:sub my_count '1' />
The snippet above subtracts 1 from what ever value is contained within my_count variable and outputs the resulting value.
The tag expects two parameters as operands for subtraction.
This tag is a self-closing one and does not set any variables of its own