Hi Dave,
The error message points into the direction that your Characteristic is of type NUMC (numeric character). I.e. it can only contain numbers 0123456789, nothing else. Can you please check your InfoObject?
In general it is certainly possible to work with alphanumeric Characteristic values as long as it is aligned with the data type of the Characteristic (i.e. data type CHAR).
Your other question re. the types, please refer to the corresponding parameters of Function Module EXIT_SAPLRRS0_001. You can double click on the Type as long as you reach the lowest level line type or structure. You have to use those types strictly to avoid any terminations.
Best regards,
Sander