welcome to the Celonis Community!
The “Reload all partitions” option is useful when you have Delta Load in place. With Delta Load, you can partition your input data based on a defined strategy.
When you then reload the datamodel from source, only the partitions that have changed since the last load are fetched from the database. With “Reload all partitions”, you can fetch (reload) all partitions from the database, no matter if they have changed or not. You can find more details about Delta Loading in the documentation: https://help.celonis.de/display/CPM45/Delta+Load
If you don’t have Delta Loading in place, the “Reload all partitions” button should behave just like the “Reload from source” button: all data will be fetched from the database.
Let me know if you have any more questions.