Some may be surprised by the 15560 message (Cannot add or drop a signature on '%' because only modules can be signed.) in one specific context: Inline tablevalued functions. <br/>
IF's are by their nature not modules but more akin to parametrized views, they have no plan of their own. Therefore it is understandable they cannot be signed and cannot be used for permission isolation. <br/>
However, both sys.sql_modules and the BOL topic for ADD SIGNATURE suggest otherwise. An IF has an entry in sys.sql_modules even though most columns do not apply, and the signing doc does not differentiate the three function types... </br>
I request documentation clarification both in http://msdn.microsoft.com/en-us/library/ms181700.aspx and http://msdn.microsoft.com/en-us/library/ms175081.aspx .