Containers

.scm and .scx are MPQ archives. The scenario itself is the archive member staredit\scenario.chk; everything else in the archive (custom sounds, graphics, whatever a map author put there) is unrelated to the scenario data. Bare .chk files open directly.

src/formats/mpq/scm.ts wraps mopaq for both directions. Reading needs PKWARE DCL decompression, because that is what StarCraft compresses nearly every file in its own archives with. Non-scenario members are held in archiveExtrasAtom and written back on save, so a map's custom content survives a round trip through the editor. The Save dialog decides how the archive is written (src/editor/save.ts, saveMap's options): PKWARE DCL compression with encryption and 4 KB sectors is Blizzard's own layout — every fixture map's scenario.chk is stored that way, flags 0x80010200 — and what every StarCraft build reads; zlib is smaller but needs 1.16.1 or Remastered; uncompressed is readable by anything. A map is written back the way it was opened unless the dialog says otherwise (loadMap reports it as scenarioInfo), and a new map gets StarEdit's layout. mopaq 1.3.0 carries the PKWARE encoder. The same dialog can leave out the sections the game never reads — ISOM, TILE, DD2, IVER, IVE2, IOWN, UPUS, SWNM, WAV (editorOnly in the section registry) — sections the registry does not know, bytes after the last section, and can merge repeated sections through combine, the same resolution the reader uses. None of that touches the scenario in memory.