From bonus

Stores the level that will be loaded after having completed a secret level. The value is hardcoded to be set to the current level + 1 when the player triggers a paletted nukebutton (any palette), if and only if the value is not already set.

If the player exits the level while from_bonus is set, he will be transported to that level in the current episode, and the value will be reset to 0.

See also secretlevel

Trivia

You might be wondering why Duke3D's secret levels still transport the player to the proper next map if you warp to them via cheats. This occurs because the exits of these maps contain a paletted nuke button as well, with the lotag set to the proper destination. However, as a result, the game will then treat the regular level as the secret map. For example, if you warp to E2L10 (Spin Cycle) via cheats, exit the map with the nuke button, then complete E2L6, you will be transported to E2L11 instead of E2L7.

Ironically, the bug could have been fixed by simply removing the hardcoded return behavior.