Within a lookup list, a double forward slash (//) will separate your actual data from comments you want in the lookup list. Nothing that appears to the right of the slashes is actually put into the data field.
Pull up the Lookup List you want to add comments to, choose the field entry to modify, and click Edit. After the data in the Enter the F2 Value box, add a space, //, and another space, and then whatever clarifying comment you want to make regarding the data. It might look something like this:
IEM // Internet Email Function
Click OK to save. In the example above, while the comment ‘Internet Email Function’ would show when the user brings up the Lookup List, only the field data ‘IEM’ would go into the actual field on the contact record.