According to the notation description in the TASTy grammar, "Prefixes of the form lower case letter*_ are for explanation of semantic content only, they can be dropped without changing the grammar". However, in practice, this isn't true.

For example, one possible Term is:

NEW                   clsType_Term                               -- new cls

However, clsType_Term isn't necessarily a Term, since it's technically read as a type tree, which can be a Type:

Additionally, Tree is mentioned in multiple places, but isn't ever formally defined.