A normal (non-partial) reload always starts by deleting all tables in the existing data model, and then runs the load script.
A partial reload will not do this. Instead it keeps all tables in the data model and then executes only Load and Select statements preceded by an Add, Merge, or Replace prefix. Other data tables are not affected by the command. The only argument denotes that the statement should be executed only during partial reloads, and should be disregarded during normal (non-partial) reloads.