Version: ELN v4.x

Tone

Write in third person. Avoid addressing the reader directly ("you") or the writer's voice ("we") -- describe what the software does, not what the user should do with it.
When a sentence needs an actor, name the actual role instead of a generic "the user" catch-all where a more specific one applies (e.g. "the designer" for Designer-role LabIMotion docs).
Step-by-step instructions stay imperative ("Click Save", not "The user clicks Save") -- the tone rule applies to descriptive/explanatory prose, not to how-to steps.

Spelling

Use American English throughout (center, gray, color, organize, label, normalize, recognize, behavior, analyze, synchronization -- not the British centre/grey/colour/organise/labelled/normalise/recognise/behaviour/analyse/synchronisation).
"Set up" (verb, two words) vs. "setup" (noun, one word): "set up a reverse proxy", but "the setup is complete".
Preserve fixed acronym/product casing exactly: SMILES, InChI, GitHub, Docker, SMARTS, ChemCLI -- never re-case these when copyediting surrounding prose.

If you're adding an image to illustrate a paragraph, place the image after the corresponding paragraph.

Keep inline representations of real UI controls -- <Btn> (from @site/src/js/btn.js) and bare FontAwesome icons -- next to the instructions that reference them, so readers can visually match prose to the interface.
When the UI changes, replace the cue with the new control's representation instead of deleting it (e.g. the v2 add/arrow button became the v3 green "+ CREATE" button, rendered as <Btn mixed={[faPlus, "CREATE"]} color={"success"}/>). Only drop a cue once the control it represents no longer exists.

Embed YouTube references with the YouTubeFrame component (@site/src/js/layout) using a youtube-nocookie.com URL, rather than a bare link -- consistent with every other video reference on the site.
Plain links remain fine for genuine reference-list entries (e.g. a "further reading" bullet, a channel link) that aren't meant to be watched inline.

Don't add "available from version X onwards"-style notes to individual pages or sections -- the version dropdown already conveys which release a page belongs to, so a blanket marker is redundant with it.
Do keep narrower sub-version caveats (e.g. "Not available in v3.12") -- those convey real information the version selector itself doesn't capture.

Link the first meaningful mention of a related concept on a page to its canonical page (e.g. the first "samples" on a page → /docs/eln/ui/elements/samples). Don't re-link later mentions on the same page, don't link a page to itself, and link at the page level rather than to an anchor. Use the surrounding prose as the link text instead of inventing new phrasing just to create a link.
Always use a relative path ([samples](elements/samples)) for links within the same docs instance -- never an absolute /docs/... one; see This Documentation for why (and a gotcha with index pages). Absolute /docs/<instance>/... paths are only for links that cross into a different docs instance (e.g. a LabIMotion page linking to an ELN page), where a relative path can't reach.

Refer to the Chemotion Electronic Lab Notebook as Chemotion ELN.
Refer to the Chemotion spectra editor for analytical data as ChemSpectra.
Refer to the repository as Chemotion Repository.
Refer to the generic-module extension as Chemotion LabIMotion (or LabIMotion on second mention within a page).