By definition, XML has a tree structure. Nodes exist in a parent-sibling relation. It is therefore a basic assumption of TEI that text also has a tree structure. This assumption is formally expressed in the model of citable text as an Ordered Hierarchy of Citation Objects. The semantics of the CTS URN is based on it.
This is, alas, not the case. Take, for example, the problem of overlapping markup. Various problems result from this situation, and it has given rise to various solutions, i.e. more or less satisfactory work-arounds. An alternative model is being developed but is unfinished and so cannot be used in Brill’s TEI efforts.
Brill therefore sticks to the solutions described in the guidelines of the TEI consortium. Milestone elements, for example, are one of several ways of dealing with non-OHCO compliance. (See also the note on how to capture the physical structure). Another example is stand-off markup as a way to deal with multiple annotations. There is some overlap between this problem and glosses and parallels, which are also multiple annotations.
Raffaele Viglianti, "Why TEI Stand-off Markup Authoring Needs Simplification", TEI 10 (2016-2019), URL https://journals.openedition.org/jtei/1838