Expression Type 
Defines how an expression is calculated. The expression type itself is determined by the implementing class.
SAP delivers the following standard expression types:
Constant (Implementing Class 0CN001)
Field of a structure (simple value request) (0SM001)
Field of a line of an internal table (0TB001)
Call function module/method (0CF001)
Boolean expression with three variables (03O001)
Simple formula (0FR001)
SAP formula interpreter (0FB001)
Truth table ("bit pattern recognition") (0PM001)
Case expression (0CE001)
Date interval (0DI001)
Random generator (0RV001)
Range expression (0RE001)
AND / OR Linking of Boolean expressions (0AO001)
Reference-like expression (0RF001)
Access to simple context (0CA001)
Simple decision tree (0TT001)