There is no reason for c_Year to be an unstored calc.
In addition, the square bracket syntax makes it look like you may be using a repeating field.
Why is that?
The expression for c_Year should just be Year( Date )
And this is a problem because ...?
When FileMaker display these little symbols, it's doing educated guessess based on the field settings.
There is only one rule for working relationships: the matchfields on the side you want to "look into" must be stored, or storeable (no container fields, global fields, summary fields, or calculation fields that reference the aforementioned field types, or related fields - or other unstored calculation fields).
If Income::date is stored, then Income::cYear can be, too, so there seems to be no problem.