"This command executes only load and select statements preceded by a replace or add prefix. Other data tables remain unaffected by the command."
So, it will only execute the load and select commands for those commands marked by the replace and add prefixes, and does not load any other data. A full load executes both sets of commands.
This is useful for when you have lots of data and only want to reload the changed portion of that data, say for 6 years of sales data when you have loaded the previous years and want to partial load the current year.
Or are you talking about variable evaluation as well?