General-purpose document processor built on top of TeX, alongside LaTeX one of the two major TeX-based typesetting systems, with its own markup syntax and a strong focus on programmable, highly-configurable document layout. Used by pandoc as one of its PDF-producing backend engines, alongside pdflatex/xelatex/lualatex. Particularly favored for complex technical documents needing fine-grained layout control that LaTeX's more macro-package-driven approach makes harder to achieve directly.