One compiled <param> element - the interned counterpart to Statifier.Document.Param. A %Param{} is not reachable only through Statifier.Machine.Donedata.params: Statifier.Machine.Content.Send and Statifier.Machine.Invoke each own a namelist and a params list of Param.t() too.

kind records which of expr/location the author wrote (:expr or :location) - Statifier.Validator.Checks.Param already guarantees exactly one is present, so this field is never ambiguous. Both kinds are read through expr/expr_location today: a <param location> compiles its path string the same way a <param expr> compiles its expression (Decision 4), which is only sound because a bound datamodel location is itself a legal value-producing expression in this engine's predicator value space. kind is kept anyway so a later read-only location-resolve switch (the epic's own Phase 4 mentioned in the plan) is a one-site change in the interpreter fold, not a lowering, Document, or validator change.

expr and expr_location follow Statifier.Machine.Donedata's own naming for the same shape: expr is the compiled Machine.expr(), expr_location is the diagnostic span for it (attribute_locations[:expr] or attribute_locations[:location], whichever attribute was written).

location is <param>'s own element span, mirroring Statifier.Document.Param's inverted-from-Assign convention (see that module's moduledoc) - it is not a compiled path, just the node's position in the source document.

No c_index: like Statifier.Machine.Donedata.expr, a <param> is not executable content, never appears in a block, and no execute_block ever runs it.