This topic is essentially for those who have experience of both DBT Codes and braille poetry layout according to BANA rules.
It has been a late addition to this version of DBT, and to fully understand the concept, you should refer to Working with Word: Word Template (BANA Braille 2014) or more specifically the Poetry section.
[linenum] or [linenumC:A] -- Handle the next word as a line number for line-numbered poetry or prose.
[linenum] treats next word from the document for use as a line number, and may cause a line break in the text. If a line number comprises more than one word, then the words may be grouped using [:] and [;], e.g.
Ordinarily, [linenum] will cause the word that is to be treated as a line number to be placed on the right margin, adjusting the right margin automatically as necessary, without otherwise interfering with text flow.
If, since the last forced line break, there has been text in the document (i.e. if the line number comes in the middle or at the end of a "paragraph") then [linenum] implies [pm3:3], which can result in a line break, so that the line number itself and the text following it will be placed on the next line. This is intended to avoid having two line numbers occupy the same physical space in the braille layout, without requiring the transcriber to plan ahead. A right margin set implicitly by [linenum] will override any later attempt on the page to reduce the right margin using [rm], unless two parameters are given to the [rm] code.
[linenum] will ordinarily adjust the right margin to leave room for the widest line number on the page (counting only line numbers set with [linenum] itself, plus two spaces. [linenum] "looks ahead" in this respect, when determining how to set the right margin. The right margin set by [linenum] is ordinarily reset at the end of each page. However, it can be reset earlier, to separate a page into different sections for line-numbering margin computations, or carried across a page boundary, by using the "override margin" feature of [rm].
[linenum] is unique among DBT codes in that it does not intefere with pending effects of other codes. So, if [linenum] and a line number appear after a [tab] code, but before the word to be placed at a tabstop, the tabstop placement will be correct even so.
[linenum] can take two optional parameters. The first specifies a column for alignment purposes. Here, -1 can be used to specify the rightmost cell on the page. The second parameter, if specified, should be l, r, or c, to specify left, right, or center alignment on this cell. Note that the paramters control where the line number will be placed. They do no affect other text on the line. Without any parameters, [linenum] is equivalent to [linenum-1:r]. [linenum] will not have any effect on any margins unless the parameters are in fact [linenum-1:r] or, equivalently, [linenum] or [linenum-1].