Store the document's item types, whole: up to 32 types, each with its fields and custom fields. null goes back to the default types (Project, Task, Note, Idea, Action). expectedRev names the revision (itemTypesRev) the change was made to: when another change landed since, 409 item_types_stale answers with the stored types and revision to re-apply the change to. Needs edit access to the whole document.