For each element, DITA2Go considers whether that element can go inside the current parent element. If not, DITA2Go uses heuristic methods based on the possible parents, level limitations, and current context.
For example, suppose your document uses a sequential structure for steps in a procedure: paragraph format Step1 for the first step, followed by several StepNext paragraphs. To convert this structure to a hierarchical DocBook structure, with paragraphs in both formats becoming <step> children of a <procedure> element, you would specify just one setting (see §26.4.2 Mapping paragraph formats to DocBook elements):
[DocBookParaTags] Step* = step
As soon as DITA2Go encounters a paragraph format that is not valid in <procedure>, the parent tag is closed.
For problem cases, you can use a DocBookLevel PI marker to explicitly set the level for an element; see §26.5.11 Specifying DocBook element levels. However, for nested lists, use a different approach; see §26.5.5 Specifying alternate ancestries for the same element.
Leaving any paragraph or character format unmapped to a parent is risky; DITA2Go might interpolate the name of a DocBook element that does not do what you want.