One compiled <transition> element - the interned counterpart to Statifier.Document.Transition, built by the compiler's transition pass.

t_index is a dense document-order identity assigned to every <transition> element in the machine, including the transition inside an <initial> element and a :history state's own default transition - both run executable content (spec 3.3, 3.6) and both need a stable index trace effects and tooling can name. Selectability is a property of where a t_index is stored, not of the index itself: Statifier.Machine.State.transitions holds only the selectable ones a state's own event matching walks; the <initial> element's transition and a history's default live in initial_transition / history_default instead, so select_transitions never has to filter this struct by kind.

source is the owning state's index - for a plain transition, the state it is a direct child of; for an <initial> element's transition or a history default, the state the <initial>/<history> element itself belongs to.

targets is [non_neg_integer()], resolved from Statifier.Document.Transition.target's id list - [] for a targetless transition, both spec-legal and distinguishable from "one target" without a sentinel (Statifier.Document.Transition's own moduledoc).

events is [[String.t()]] - one dot-split token list per whitespace- separated descriptor. Statifier.Document.Transition.event already did the first (whitespace) split at lowering time; this struct does the second (dot) split, so event="a.b c" compiles to [["a", "b"], ["c"]] and event matching walks tokens without ever splitting a string at runtime.

cond is Machine.expr() | nil, compiled through Statifier.Compiler.Expressions.compile/3 with owner {:transition, t_index}. cond_location is the cond attribute's own value span (attribute_locations[:cond]), falling back to the transition's own location when the attribute has no recorded span - retained so a runtime cond failure can point inside the expression (ADR-0014 item 4). Both are nil exactly when cond was not written.

attribute_locations is Statifier.Document.Transition's own map, carried through unchanged rather than distilled into per-attribute *_location fields - the escape hatch Statifier.Machine.Invoke's moduledoc describes, applied here because event, target and type each have a diagnostic use (an attribute-level hover target) and none has a distinct enough one to pay for a field of its own. cond_location above is the deliberate exception, retained because it also carries a fallback the raw map does not: the transition's own location when cond was written without a recorded span.

The map keeps Statifier.Document's key-presence contract verbatim: an entry exists only for an attribute the author actually wrote, so Map.has_key?(transition.attribute_locations, :type) is the "was type written" question that type's own value cannot answer once lowering has applied the :external default. %{} when the element wrote no attributes at all, and on the synthesized initial transition (Statifier.Interpreter), which no author wrote.

content is [c_index], the transition's own executable content in document order - [] until the compiler's executable-content pass populates it.

t_index is non_neg_integer() | nil: nil names exactly one producer, the synthesized initial transition Statifier.Interpreter.initialize/2 builds when the root wrote no <initial> element - not a document element, so it has no document-order index.