Document-unique identifier. Constrained to a leading letter or underscore followed by letters, digits, underscore, hyphen or dot, so that an id is usable unmodified as an identifier in every target language an SDK might generate — and so it never needs escaping in an expression. An expression in the HarnessXML expression language. Interpolated forms are wrapped in ${ }. See spec/v1.0 §Expressions. Not validated here — well-formedness is HX-3101, checked by the validator. ISO 8601 duration, e.g. PT30S, PT5M, PT2H, P1D. Months and years are forbidden (HX-3401) because their length is not fixed and a scheduler cannot resolve them deterministically. CLOSED enumeration. A new node type is a minor-version addition (an implementation meeting an unknown type MUST fail with HX-1003, never skip the node). See GOVERNANCE §5. Generic unit of executable work. Invocation of a model resource. Pure function of its inputs. No side effects, therefore freely retryable and cacheable. Evaluates cases and routes control to exactly one successor. Repeats a body node. Fans control out to all successors at once. Join. Waits per its joinPolicy before releasing control. Invokes another HarnessXML document. Boundary node producing data into the workflow. Boundary node consuming data out of the workflow. Blocks on a duration or an external event. Human-in-the-loop. Blocks for an out-of-band decision. Typed relationship. The type determines what the edge means to the scheduler — this is the core of the execution model, not decoration. Ordering. Target becomes READY when the source reaches a terminal successful state. Dataflow. Implies control, and additionally binds an output port to an input port. Must-complete-before with no data transfer and no implied success — satisfied by any terminal state. Taken only when the source reaches FAILED after retries are exhausted. Rollback path. Traversed backwards during compensation, never during forward execution. Wait for every incoming edge. The default. Release on the first. Siblings still in flight are CANCELLED. Release when @quorum incoming edges are satisfied. Port type. OPEN-ENDED: the listed values are the interoperable core, and an implementation MAY use a URI or a qualified name for a domain type. Compatibility across a data edge is HX-3201. Vendor extension point. See GOVERNANCE §6. required="true" means a runtime that does not understand this extension MUST refuse to execute the document (HX-1004) rather than execute it differently. Requested execution isolation, e.g. "process", "container", "vm". Advisory; a runtime that cannot honour it MUST fail rather than downgrade silently (HX-4201). Retry policy. Absent means no retry. A node with side effects that is not idempotent SHOULD declare idempotent="false" on the node so that a runtime can refuse to retry it rather than duplicate the effect. Space-separated error classes to retry on. Absent means retry any failure. Literal or expression bound directly, for an input not fed by a data edge. Where this document came from. The point of recording it is that a HarnessXML document is usually GENERATED from a visual graph — and an execution trace is only auditable if it can be tied back to the design that produced it. A capability the workflow needs but does not contain: a model endpoint, a database, a robot arm. Declared once and referenced by nodes, so that moving a workflow between environments is a change to the resource block and nothing else. A REFERENCE to a secret, never the secret. A document containing a literal credential is invalid (HX-3501) — the specification refuses to make leaking one convenient. Content digest, e.g. "sha256:abc…". Makes a run reproducible and an audit trail meaningful. Conditional execution of THIS node. If the expression is false the node enters SKIPPED, which is a terminal successful state — successors on control edges still run. This is distinct from a decision node, which routes between alternatives. Required on type="decision", forbidden elsewhere (HX-2201). Cases are evaluated IN DOCUMENT ORDER and the first true one wins — ordering is normative, so that a decision is deterministic. Required on type="loop", forbidden elsewhere (HX-2202). maxIterations is REQUIRED: an unbounded loop in a workflow that may run unattended is a defect, not a feature. Required on type="subworkflow", forbidden elsewhere (HX-2203). Required on type="wait", forbidden elsewhere (HX-2204). Implementation handle the runtime resolves — a function name, an image reference, a URI. Deliberately opaque to the specification: HarnessXML describes the WORKFLOW, and stops at the boundary of what a step does. False means a runtime MUST NOT retry this node automatically (HX-3301 if combined with a retry policy). Marks this node as the compensating action for another node. Required when type="data" (HX-2301). Required when type="data" (HX-2301). Guard on traversal. The edge is taken only if this evaluates true. Explicit entry node. If absent, the entry set is every node with no incoming control, data or dependency edge (HX-3001 if that set is empty).