How to tell if a column is a valid lookup reference?

Question

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?

Answer 1

I'm not sure this translates to SP2007 100%, but from a SP2010 perspective the following columns will show up when creating a new Lookup field:

SPFieldText, SPFieldDateTime, SPFieldNumber, SPFieldType.Counter (i.e. ID), SPFieldComputed (EnabledLookup property has to be True), and SPFieldCalculated (OutputType property has to be SPFieldType.Text).