Siemens SIMATIC PCS 7 Operation And Maintenance page 87

Downloading to PLCSIM or a test CPU
You want to test your program changes in advance in PLCSIM or on a test CPU.
If you use "Changes only" mode for this download, the delta download capability of the plant
CPU will be lost due to the different time stamps.
You can maintain delta download capability by selecting "To test CPU (entire program)"
mode for the download.
Archiving
Make sure you archive the latest project version after every download. Otherwise, in some
circumstance it may not be possible to perform a delta download with a retrieved project due
to time stamps that have changed over time.
Compendium Part D - Operation and Maintenance (V9.0)
Operating Manual, 02/2018, A5E43228948-AA
AS and OS expansions
7.2 Program changes
87