Let R be a ring, K, M be R-modules, L a uniserial R-module, and X a submodule of L. The triple (K, L, M ) is said to be X-sub-exact at L if the sequence K → X → M is exact. Let σ(K, L, M ) is a set of all submodules Y of L such that (K, L, M ) is Y -sub-exact. The sub-exact sequence is a generalization of an exact sequence. We collect all triple (K, L, M ) such that (K, L, M ) is an X-sub exact sequence, where X is a maximal element of σ(K, L, M ). In a uniserial module, all submodules can be compared under inclusion. So, we can find the maximal element of σ(K, L, M ). In this paper, we prove that the set σ(K, L, M ) form a category, and we denoted it by C L . Furthermore, we prove that C Y is a full subcategory of C L , for every submodule Y of L. Next, we show that if L is a uniserial module, then C L is a pre-additive category. Every morphism in C L has kernel under some conditions. Since a module factor of L is not a submodule of L, every morphism in a category C L does not have a cokernel. So, C L is not an abelian category. Moreover, we investigate a monic X-sub-exact and an epic X-sub-exact sequence. We prove that the triple (K, L, M ) is a monic X-sub-exact if and only if the triple Z-modules (Hom R (N, K), Hom R (N, L), Hom R (N, M )) is a monic Hom R (N, X)-sub-exact sequence, for all R-modules N . Furthermore, the triple (K, L, M ) is an epic X-sub-exact if and only if the triple Z-modules (Hom R (M, N ), Hom R (L, N ), Hom R (K, N )) is a monic Hom R (X, N )-sub-exact, for all R-module N .