It seems that some symmetry between messages and terms are missing, intentionally or not, and I will like to understand better the "why". Let's see the example from the documentation:

login-input = Predefined value
 .placeholder = email@example.com
 .aria-label = Login input value
 .title = Type your login email
-brand-name = Aurora
 .gender = feminine
 .anadrome = Arorua
 
x = { login-input.title }
y = { -brand-name.anadrome }

In this example, the message login-input can have attributes that are placeables, while -brand-name term can only have metadata (values for selection and such). The specific parsing error is: E0019: Attributes of terms cannot be used as placeables.

I might be missing some fundamental design principle that makes terms a lot more more restrictive than messages, but in principle, it looks to me that the ability to group information (of various kind) should be common for both. This issue is also in the same realm as #338.

cc @gonzalocordero