Mark,
The policies that PMI describes in the CCRS Handbook (
www.project-management-prepcast.com/ccrshb
) are all focused on continued learning for project managers who are PMI certified. The focus on these learning activities has to be within what PMI calls "domain-related services". So if your are a RMP certified then you can earn PDUs for risk related activities, for SP you can earn PDUs in scheduling/time management or if you are Agile certified then your activities must be related to agile practices.
In short: You cannot earn PDUs for activities that are not related to your certificate.
In your specific example you mention that you are teaching classes. This would not fall under "Volunteering", but under "Give a Presentation". Volunteering is reserved especially for volunteer activities for project-management organizations like serving as a board-member for a local PMI or chapters or the ASAPM. However, if as you say, parts of the class you are teaching are indeed relevant to project management then you can claim PDUs for presenting. You must determine yourself how much that is.