Appendices-Terms-r-value

A R-value is any initialized variable, expression, constant, or conditional variable. The expressions that count include function calls, property references (for the get accessor, operator calls, etc. Please note that an uninitialized var doesn't count as an R-value until it has a value.

var i% ' not an R-value until it has a value
i% = 3 ' now you can treat it as an R-value