Simplicity. A small number of elements with very few variations. In the above example, bubbles represent Activities, the arrows the pathways that can be taken through them.
Plain language – what I call ‘Anglo-Saxon’ – as spoken by ordinary people. Active verbs, combined where needed with a single noun. Together they express the desired outcome of the Activity.
Some rigour, but not rigidity. The aim is to communicate the what to a competent and intelligent human being, not a robot or a computer. A bit of fuzziness is tolerable if it helps to get the idea across in an uncluttered way.