Need help understanding the tuple relational calculus We say that a query in a TRC is a statement of the form $\{T: P(T)\}$, where $T$ is a tuple variable and $P(T)$ is a formula. 

Now my question is, what are the possible tuples $T$ to be tested i.e., what are the types and values for tested tuples.

As far as I know in set theory and in particular in set builder notation the variables should come from another well-defined set.
From Wikipedia: 

A tuple is of the form $T=(D,R,h)$ where $D$ is the domain, $R$ is a set of relation names, and $h$ is a mapping that associates a column name to each relation in $R$. To represent a tuple, we can see $D$ as the set of keys, and $R$ the table associated with those set of keys, and $h$ the column names of the table.