Ideally, levels (static geometry) are authored in an efficient manner either as modular instanced components that can be culled efficiently at runtime (based on camera view frustum or Occlusion) or as discreet "chunks" that factor in where the player can navigate and thus strip out unnecessary geometry.
This, however, is not always the case. Sometimes it is more effective for artists to author environments as singular components and have this data processed offline as part of a build step.
In order to do the latter, typically, the content creators are provided with some kind of interface to how the resulting level will be split up, most often a grid composed of uniform dimensions.
This video demonstrates one such process working within Maya. Simple geometry is used in this case, however the process would be just as effective on a complex level. The grid used to do the cutting can contain non-uniform dimensions (as shown). All level objects are cut up as part of the process so that the chunks do not overlap one another and align perfectly at runtime - note the green, red and blue objects.
Authored in Python.
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