First time filing a feature request against TEI, so please be gentle.
Attributes should not be used to encode multiple, space-delimited values. This is a denormalized approach that should be reconsidered.
For example, the "@active", "@mutual", and "@passive" attributes of <relation> element accommodate "a list of identifier values for participant or participant groups". Burying such information in a space-delimited attribute value make it hard to extract, e.g. with XPath.
Instead, a child <ref> element or elements could be used to encode this information.