BPMN21 — The spec should clearly state what visual features are available for extensions and which are restricted to core spec

Right now there is no clear separation on which graphical features (like line thickness, type of lines, color, etc) vendors are free to use as extensions, and which are restricted for the specification to use.

We should define which features we want to use in the spec and which ones vendors are free to use. The reason for this is that if a vendor chooses line thickness for some visual extension and in a revision we choose to use the same feature, the vendor is forced to change to adapt (most importantly end-users).

For example: it is clear that color is open for vendor extensions, and we should pick lines (thickness, dotted, etc) as restricted for our use.