Create, retype, or remove a dependency between two items — work items (epics, features, stories, bugs, tasks, requirements) and/or portfolio initiatives. Products, OKRs, key results and super-initiatives cannot have dependencies and are rejected (422) on create/retype; remove is allowed regardless of type. Upsert — creates the relation or changes its type; remove: true deletes it (idempotent — removing an absent dependency returns 200 with removed=false). Directional from the path item: "item <type> relatedItem". Relation types: blocks, blocked by, related. Read dependencies via get_item fields=all.