Displaying of Logs in ABAP List-Based PI Sheets 
When you display a PI sheet, the system creates the following logs:
A display log which documents errors found when the PI sheet was edited for display
(for example, that a long text referred to in the PI sheet does not exist in the system)
A loading log which documents errors found when the data for the individual process instruction was loaded (for example, that the message category requested in a process instruction does not exist)
When you maintain a PI sheet, the following additional logs are created:
A maintenance log which documents errors discovered during data maintenance
A calculation log which documents all errors found during calculations
Whenever new entries are made in a log, a corresponding system message is displayed.
The display log always contains the information relevant to the PI sheet section currently displayed. The data in all other logs is retained until you leave the PI sheet or change from display to maintenance or vice versa.