A partial reload happens in the context of the resident, in-memory data model. It tells Qlik not to drop the complete data model when doing the script reload and do a full reload, but keep some data tables and add / replace data in other tables.
An incremental (or sometimes called Delta) LOAD is a more basic concept which is not limited to the context of the in-memory data model.
For example, it's often used when creating QVDs from source data base tables. You may want to only query the source DB for new, updated or deleted records (in comparison to an existing QVD), not pulling all data in. But your QVD should finally be updated to represent the current state of the DB table.
Here, you can't use a partial LOAD because there is in general no data model that will be kept in memory till the next reload.