You are allowed to define your own variables and functions. As a name of a variable or a function can be a name consists of letters, digits or an underline character but the first character must be a letter. From version 0.8.3 names are case-sensitive that means 'name' is not the same as 'NaMe'. For example these are correct names: a, b, c3, MyName, etc. During defining variables and functions you can call another variables and functions but you cannot call a variable or a function twice. Recurrence calling are not allowed.
Functions can get maximum 9 parameters. You have to specify how many parameters your function takes and you can refer to them by using the variable 'x' with a number from 1 to 9 ('x' without a number means 'x1'). For example you can define a function 'xy' as 'x1*x2' (of course without apostrophes), set the 'parameters' on 2 and then put into the edit box an expression 'xy(10;20)', the result will be 200. Note that we use a semicolon to separate arguments in a function.