How to find out about the variable name phys.Solid.sl (in an error message)

I get an error message:
"Tensor inner product failed. Tensor product must a least have rank equal to 2."
"phys.Solid.sl:item.alpha"

How can I find out where and how the variable "phys.Solid.sl:item.alpha" is defined?
Where is it set? What does it contain?

I checked "equation view" within the different knots .... but no variable name started with phys.
I inserted the variable name in the "search" of the help utility, but nothing was reported.