1. Terms

Term	Meaning
Design file	A `*.design` file. One UTF-8 JSON object holding one design document.
Assets folder	The folder `<name>.design.assets/` next to the design file. It holds image and font files named by content hash (section 8).
Document	The design as a whole: metadata, pages, nodes, styles, variables, libraries, flows, comments, versions.
Tree	The `tree` member: the document as it was at the last consolidation (section 4).
WAL	Write-append list: the `wal` member, the events made since the last consolidation (section 5).
Event	One entry of the WAL: one change, made by one actor at one moment.
Envelope	The members every event has: `seq`, `at`, `by`, `op`, and optionally `group` and `note` (section 5.1).
Tree hash	`sha256:<hex>`: the SHA-256 of the canonical JSON of a folded tree. It names a state of the document (section 6.9).
Epoch	The `ep-` id a consolidation gives the tree it writes; the `wal.base` marker carries it (section 5.4).
Operation (op)	The kind of change an event makes, for example `node.update` (section 5.11).
Payload	The members of an event besides the envelope.
Actor	The person or tool that made a change; its display name is the event’s `by` (section 5.8).
Group	Consecutive events made by one transaction: one undo step, one history entry (section 5.7).
Fold	Apply events to a state, in file order, to compute the current document (section 6.1).
Coalescing	Merging a new event into the previous unsaved event when both belong to one continuous edit (section 6.2).
Consolidation	Folding the whole WAL into the tree and emptying the WAL (section 7).
Node	One layer: a frame, a shape, a text, an instance, and so on. Pages are not nodes (section 4.2).
Node map	`tree.nodes`: every node of every page, keyed by id, flat (section 4.3).
Parent	The node or page a node belongs to, named by its `parent` member.
Fractional index	The `index` string that orders a node among its siblings (section 4.3).
Stored property	A property written in the file.
Derived value	A value the core computes and never writes: auto layout positions, hug sizes, group bounds, text lines, instance sublayers (section 4.5).
Default	The value of a property that is not written. Writers leave out properties that equal their default.
Merge patch	A partial object applied to a stored object as in RFC 7396: members are set, `null` removes, nested objects merge, arrays are replaced (section 5.9).
Binding	A reference from a property to a variable, in `boundVariables` (section 4.10).
Main component	A `COMPONENT` node. Instances show a copy of it.
Instance sublayer	A layer inside an instance. It is derived from the main component; it has a derived id and is not stored (section 4.8).
Override path	The ids from an instance down to one of its sublayers, joined by `/` (section 4.8).
Library	Another `.design` file whose components, styles and variables this file uses (section 4.11).
Asset	An image or font file in the assets folder, named `sha256-<hex>.<ext>` (section 8).
Conflict	Two events from two merged branches that set the same property of the same object (section 6.4).
Orphan	A node whose parent no longer exists (section 6.5).
Reader, writer	A program that reads or writes design files: the extension, `ud`, an agent, a script.