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The current level will terminated. The engine cache holding level entity files is purged. All current entities in the old level are removed, all entity pointers referring to them cannot be used any longer.
All actions assigned to or functions called by level entities - that means all functions in which the my pointer is not zero - are automatically terminated at their next wait instruction.
The game script itself will keep running and won't change, even if a different script was assigned by WED to the new level.
One frame cycle after this instruction the level is loaded, two frame cycles after the instruction the local entities in the new map are created and their actions are started. Wait two frame cycles (wait(2);) before modifying the level, for instance by creating entities.
When giving a HMP or MDL file name, the new level contains this terrain or model only. When using an empty string (""), an empty level will be loaded.
During the load process, the on_level event can be used to display a progress bar or animation on a panel.
One frame after level loading the last_error variable indicates loading errors.



From the almighty manual