AT&T 6300 Programmer's Manual page 647

Example
Advanced Features
110 DEF USRO=&H8000
115 rAssumes user gave IM:32767
120 X=5
130 Y = USRO(X)
140 PRINT Y
The type (numeric or string) of the variable
receiving the function call must be consistent
with that of the argument passed.
B-2!