We study the category of \(\textbf{P}\) -equivariant modules over the infinite variable polynomial ring, where \(\textbf{P}\) denotes the subgroup of the infinite general linear group \(\textbf{GL}(\textbf{C}^\infty )\) consisting of elements fixing a flag in \(\textbf{C}^\infty \) with each graded piece infinite-dimensional. We decompose the category into simpler pieces that can be described combinatorially, and prove a number of finiteness results, such as finite generation of local cohomology and rationality of Hilbert series. Furthermore, we show that this category is equivalent to the category of representations of a particular combinatorial category generalizing \(\textbf{FI}\) .