Content can be auto-fetched for groups of records from some other applications. This is not a sync mechanism.
Multiple watched groups are allowed though be mindful heavy use of this feature may affect overall performance, i.e. the feature is not designed as a fully-synced system but rather for importing/updating some DEVONthink data.
A receiving container for the group's data is always added at root level in a document (and should not be moved elsewhere). This is for necessary design considerations re performance.
Refresh timing: all watched folders are refreshed when the TBX document is opened. Then, while the document remains open, the folders are refreshed periodically on their own cycle. The frequency of that cycle is not documented and no force refresh mechanism is known.
There is currently one form of watched group: