Add the term "behavior, implementation-specific"

standard/portability-issues.md

@@ -51,6 +51,12 @@ A conforming implementation is required to document its choice of behavior in ea
 1. The behavior of the `fixed` statement if the array expression is `null` or if the array has zero elements. ([§24.7](unsafe-code.md#247-the-fixed-statement))
 1. The behavior of the `fixed` statement if the string expression is `null`. ([§24.7](unsafe-code.md#247-the-fixed-statement))
 
+## §Imp-Spec-Behavior Implementation-specific behavior
+
+The behaviors listed in this subclause need not be documented. The following are implementation-specific:
+
+1. How a nullable context is specified external to the source code to which it applies. ([§8.9.4.1](types.md#8941-general))
+
 ## B.4 Unspecified behavior
 
 1. The time at which the finalizer (if any) for an object is run, once that object has become eligible for finalization ([§7.9](basic-concepts.md#79-automatic-memory-management)).

standard/terms-and-definitions.md

@@ -8,6 +8,7 @@ For the purposes of this specification, the following definitions apply. Other t
 - **assembly** – one or more files output by the compiler as a result of program compilation
 - **behavior** – external appearance or action
 - **behavior, implementation-defined** – unspecified behavior where each implementation documents how the choice is made
+- **behavior, implementation-specific** – optional behavior or feature support that does not invalidate conformance and for which no documentation is required
 - **behavior, undefined** – behavior, upon use of a non-portable or erroneous construct or of erroneous data, for which this specification imposes no requirements
 - **behavior, unspecified** – behavior where this specification provides two or more possibilities and imposes no further requirements on which is chosen in any instance
 - **character** (when used without a qualifier)