Definition:Classes of WFFs/Plain WFF

Definition
A plain WFF of predicate calculus is a WFF with no parameters.

Thus $$WFF \left({\mathcal P, \varnothing}\right)$$ is the set of all plain WFFs formed from $$\mathcal P$$.

Note that a WFF with parameters from $\mathcal K$ is, by definition, a WFF whose parameters are all in $$\mathcal K$$.

That is, none of its parameters come from outside of $$\mathcal K$$.

Hence a plain WFF is a WFF with parameters from $\mathcal K$ for all $$\mathcal K$$.