For the moment during the entity import, we can compute the values for fields and save those as a result. In a case the "process" stage of an import returned "NULL" the value will be emptied. The other case - new value or set of values that will be saved for an entity, overriding the previous ones.
Imagine we have the "field_tags" for an entity and we doing an import into it. The type of field is "entity_reference" and it's multivalued. Initially, the source contains two tags: "a" and "b". The second import comes from the source that has "c" and "d" tags. What will be the result of two migrations? It'll be "c" and "d" unless process plugins haven't implemented some specific handling.
This issue is about allowing to merge the values among the incremental imports for specifically configured fields.
