There are several ways in which the lg element may be used
within your document as a whole, depending on how the poem(s) in
question are situated:
The poem as a single text. If you are treating the poem
itself as the object of interest, rather than the document from which
it is taken, you may choose to omit most or all of the surrounding
documentary material (for instance, if the poem is being transcribed
from a magazine or an anthology). In this case, it may make most sense
to use a separate text element for each poem, with the poem
itself encoded as lg within the body of the
text element.
The poem as a subdivision of a collection. If you are encoding a
collection of poems for which the original document is an object of
interest, then each poem constitutes a subdivision of the whole
work. In this case, each poem should be encoded with div
(which can enclose the poem and any accompanying material that appears
such as an introductory note, etc.) with lg inside the
div.
The poem as an intervention in some other genre. For poems which
appear embedded within other textual structures, such as a novel or
letter, or when quoted, use text or simply lg. The
chief practical difference here is that text may contain
div, so if the poem has subdivisions text is a better
choice. However, it’s rare for a long, subdivided poem to be quoted at
length in a context like this, so lg will probably be fine in
almost all cases.