“A text” in the sense of literary theory is a coherent arrangement of signs that can be read to transmit meaning.
“Text” more usually refers to a composition of letters selected from a fixed alphabet, typically in writing or in print.
“Plain text” is often used in software to mean something like “data that will be human-readable through a relatively standard text renderer”. This is often nice and also incredibly fraught.1
Aria Desires, “Text Rendering Hates You,” Faultlore, September 28, 2019, https://gankra.github.io/blah/text-hates-you.