The answer is: structure.
The division of a poem has to do with its structure. It is written in stanzas and lines. The use of rhythm, the lenght, the placement and the grouping of the lines and the stanzas are also important to divide the poem and to express the ideas the writer wants to.
The development, the happening, the incident, the occurrence, the situation. The affair, the encounter, the meeting, the moment. the episode, the problem and solution. Others have mentioned these or their ilk: the chapter, the passage, the story, the period, the attack, the bout, the installment, the phase.
