<p>This note offers an unusual approach of studying a class of modules inasmuch as it is investigating a subclass of the category of modules over a valuation domain. This class is far from being a full subcategory, it is not even a category. Our concern is the subclass consisting of modules of projective dimension not exceeding one, admitting only morphisms whose kernels and co-kernels are also objects in this subclass. This class is still tractable, several features are in general simpler than in module categories, but lots of familiar properties are lost. A number of results on modules in this class have resemblance to those on modules over rank one discrete valuation domains (where the global dimension is 1). Our study is considerably simplified by taking advantage of the general theory of modules over valuation domains available in the literature, e.g. in Fuchs and Salce (<CitationRef CitationID="CR12">1985</CitationRef>)- Fuchs and Salce (<CitationRef CitationID="CR13">2001</CitationRef>). Our main goal is to establish the basic features and have a closer look at direct sums of cyclic modules, injectivity, and pure-injectivity, but we do <i>not</i> wish to enter into an in-depth study of these properties.</p>