An arbitrary number, already defined, as you say, by indentation.
See, for example: Blog post: Scientific refereeing using Bike Outliner - Bike Outliner - Hog Bay Software Support
and particularly:
Absolute vs. relative hierarchy in document markup languages
it is the context of a node that determines what its level in the hierarchy is, not the node itself.
i.e. to be a header of level N is not a quality of the row itself – it’s a (nesting) relationship to other rows, and to the scope of the subdocument which you are considering or using.
Once Bike has flagged a row as a header, its effective level depends on:
- where it is,
- what sub-outline / sub-document you are extracting,
- and what you are defining as the outermost starting level for the purposes of formatting that sub-document, for a specific use.
(and, in the Bike 2.0 stylesheet system, you will be able to define different visual styles for data-type="heading" rows at different levels of nesting).