Domain Dim

Key _Domain Dim in Signal refers to the dimensionality of this function's domain.
Since the function maps u variables x₁ , x₂ , ... , xᵤ, the dimensionality referred to here equals u
In the special case where u is undetermined in the sense that the function can process any number of variables, the _Domain Dim is set notationally to -1.

Note this key is prefixed by _, which indicates it is regarded as a read-only part of the object's data and therefore its associated value cannot be edited.
For that reason it does not appear as part of the input data.
It carries informational only value for the user and appears as part of its contents.
Also it completely depends on the remaining object contents and therefore is never part of the exported object's data.