This paper focuses on surface defects of mechanical joint composed of two plane surfaces of two carters assembly. The originality of the proposed study is to manage both form errors and sliding surfaces complexities in a Computer Aided Tolerancing process. The assembly procedure is introduced in a tolerancing analysis process in order to assess designer's tolerance choices. It's also considered the angular orientation of the carter's plane surface, corresponding to the mobility of the assembly. The domain of relative positions according to this positioning parameter is computed. This one, called "mobility precision domain", is defined for a particular sliding assembly.