2.1.1161 Part 1 Section 19.5.47, inkTgt (Ink Target)

a.   The standard places no restrictions on what type of shape the spid attribute references.

PowerPoint requires the spid attribute to reference a valid ink shape.  PowerPoint will read inkTgt elements, but on read the ink shape will be converted to a contentPart and its corresponding inkTgt will be written out as a spTgt.