Clarify how input validators report their results

spec/2025-09.md

@@ -670,7 +670,7 @@ Their goal is to ensure the integrity and quality of the test data and validatio
 
 The files under `invalid_input` are invalid inputs.
 Unlike in `sample` and `secret`, there are no `.ans` files.
-Each `tc.in` under `invalid_input` must be rejected by at least one input validator.
+Each `tc.in` under `invalid_input` must be rejected by at least one input validator. (Other input validators may reject the test case, accept it, or fail to operate properly.)
 
 The relevant settings from [Test Case Configuration](#test-case-configuration) (`input_validator_args`, and `description`) can be set in `tc.yaml` for one test case, or `test_group.yaml` for all test cases in the same directory.
 
@@ -985,12 +985,25 @@ first pass will be used.
 The input validator may output debug information on stdout and stderr.
 This information may be displayed to the user upon invocation of the validator.
 
-### Exit codes
+### Reporting Validation Results
 
-The input validator must exit with code 42 on successful validation.
-Any other exit code means that the input file could not be confirmed as valid.
+The input validator can accept a test case as valid, reject a test case, or fail to operate properly. Although validation fails in both of the latter cases, they should be clearly distinguished in any diagnostics reported to the user.
 
-#### Dependencies
+The Checktestdata and VIVA APIs define their mechanisms for reporting validation results. Input validator programs report a result via exit code.
+
+#### Exit Codes for Input Validator Programs
+
+An input validator program must return with one of the following codes:
+- the input validator must exit with code 42 on successful validation;
+- the validator must exit with code 43 to reject the test case.
+
+The purpose of these somewhat exotic exit codes is to avoid conflict with other exit codes that results when the validator crashes.
+For instance, if the validator is written in Java, any unhandled exception results in the program crashing with an exit code of 1,
+making it unsuitable to assign a meaning to this exit code.
+
+An input validator program that does not compile or run, crashes, or returns with any exit code other than 42 or 43 fails to operate properly.
+
+### Dependencies
 
 The validator **must not** read any files outside those defined in the Invocation section.
 Its result **must** depend only on these files and the arguments.
@@ -1166,9 +1179,6 @@ A validator program must report its judgement by exiting with specific exit code
 
 Any other exit code, **including 0**, indicates that the validator did not operate properly,
 and the judging system invoking the validator must take measures to report this to contest personnel.
-The purpose of these somewhat exotic exit codes is to avoid conflict with other exit codes that results when the validator crashes.
-For instance, if the validator is written in Java, any unhandled exception results in the program crashing with an exit code of 1,
-making it unsuitable to assign a judgement meaning to this exit code.
 
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