BOM is relevant to plant maintenance

What causes this issue?

Due to the definition of the BOM usage, the BOM is relevant to plant
maintenance.
In a plant maintenance BOM, there are items which are only used for
defining the structure of the equipment (PM assemblies). You enter
these items using item category PM structure element. The system does
not check the plant for these materials.

Error message extract from SAP system. Copyright SAP SE.