The explicit definition of variability in software product lines is a key difference between the development of single software systems and software product line engineering. More and more companies maintain several software product lines which focus on different types of products, market segments, and/or domains. Those product lines typically share commonalities and variability. The companies thus face the problem of managing communality and variability across different product lines.

In this paper we identify essential requirements for the documentation of requirements variability across product lines. We propose a meta model for structuring the variability information, sketch a prototypical realisation for managing variability across product lines in DOORS, and illustrate the use of the meta model in a small example. We further report on experiences made with the proposed variability modelling approach.