The paper identifies some recurrent forms of ‘software-specific’ vagueness in the context of software engineering work practices as a set of conditions for establishing shared understanding between engineers and users on the features and possible uses of the system. Relying in particular on about 20 hours of audio-video recording of meetings between the software engineers and users at three different sites, the paper identifies three recurrent patterns of ‘software-specific’ vagueness: (i) “openness versus completeness”, that is, the extent to which the software-based modeling tool requirement of internal completeness hampers the flexibility crucial to its function; (ii) “representation versus coordination”, that is, the degree of recognizability of system features as management support functions of control with respect to line work management functions and (iii) “object orientation versus procedural orientation”, that is, the order that the model imposes on interaction and the representations of interaction located within the object instead of on a procedural level as in e.g. issue lists. Contrary to expectations concerning software engineering when it is viewed as a matter of logic, the establishment of a common ground of reference between engineers and users does not rely on making instructions about the software more accurate, more detailed or more specific. Rather, the paper argues that the vagueness of many expressions, specifically with regard to openness, coordination functions and procedural-orientation of the software, helps build a shared understanding.