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Welcome to the DHIS2 Community of Practice!
You mentioned two different things in your question, “program rules” and “Validation rules”… If you are using a tracker program then you’ll be using program rules, you’re welcome to read more about program rules in the documentation: Programs - DHIS2 Documentation
If you’re on the aggregate side, creating datasets, then you’d use validation rules: Metadata - DHIS2 Documentation
Please explain further your inquiry because the topic was posted without details only the title. Thanks!