Stephanie C. answered 06/19/19
B.A. in English, University of Arizona, specializing in literature
A transition establishes the connection between two topics or ideas. A digression breaks from the main subject or narrative.
Say you're writing about the economics of the Roman Empire and you introduce several topics, like agricultural production, manufacturing, trade, and use of slave labor. Midway through your paper, after you've discussed trade relations among various parts of the empire, you want to TRANSITION to a discussion of slave labor. One way to transition would be discussing how slaves were traded through the empire and then get into the types of labor they performed.
To signal a reader that you were going to DIGRESS on the morality of slavery or perhaps how it differed in the Roman era versus the period of the transatlantic slave trade, you would maybe use a short paragraph to explain that you planned to discuss this side topic briefly and why it was important to include although it isn't directly relevant to the economics of the Roman Empire.
Literarydevices dot net has some good resources and examples you may wish to read.