Currently, the only out-of-the-box ways to attach multiple customer assets to a work order is through work order incident--create one for each asset--or work order service task--create one maintenance task for each customer asset.
However, this method of operation is a frequent source of confusion for customers as work order incidents implicate (and technically require) a resolution, and the label "work order incident" indicates that this is a lot more than simply adding information to the work order. Work order incidents can be created automatically via agreement incidents when the work order is generated via an agreement.
In my recent experiences deploying the agreement module for customers, it was a unanimous feedback that it would be much more straightforward and simpler for the customer to create an "agreement asset" relationship table and "work order asset" relationship table. We've achieved this through customization and synced the tables via Power Automate flows, but this seems like a common enough request from my customers that I wonder if there might be enough interest to create a "multi-asset mode for work orders and agreements" functionality that would be natively available from Field Service. The relationship table would replace the OOB lookup field and integrate the OOB lookup field from cases with the work order relationship table so that we are able to use this in multiple scenarios without customization.
Regards,
Justine