When creating a lookup column in SharePoint 2007 the user interface restricts the available columns to a subset of all columns in the list. How does SharePoint decide which columns are available?
The accepted answer to this question: Lookup from list column? states that it is only text columns, but that answer is only partially correct as the available selections include the non-text field ID (type Counter), as well as the "Title (linked to item)" option.
Does it add the ID and LinkTitle fields just because it knows they're special?