The developer can decide whether something is executed in parallel or synchronous.
Wendelin and ERP5 use an internal activity tool called portal_activities
for dispatching jobs (or in ERP5 language "activities") to a respective
node in a system or a cluster.
Please look at how to use
parallel processing (coming soon) for a walkthrough on how activities can be launched
in parallel or synchronous in Wendelin.
For information beyond Wendelin please refer to How To Use CMFActivity Tool
(from the ERP5 Documentation).