A number is a post-fix expression consisting of numeric values, variable values and operators.
A numeric value is anything starting with a digit. 0x prefixes a hexadecimal value, 0 prefixes an octal value otherwise the value is interpreted as decimal.
Variables are single upper case characters in the range A-Z, thus 26 variables are available.
Numeric values and variables are pushed onto a stack which can store up to 248 numbers.
Operators are single characters not identified as numeric values or variables. They operate on the stack combining elements.
Use -h operators for more information.