This video provides a comprehensive understanding of how to apply validation rules during the document posting process in SAP S/4HANA Financial Accounting (FI). Here are the key points:

1. Validation Rules: The video explains that validation rules are used to check and validate the values entered during the document posting process. The system validates these values according to the validation rules and issues a warning or error message during the document posting.
2. Creating a Validation Rule: The presenter demonstrates how to create a validation rule in the system. He creates a validation rule for his company code ‘HA01’ in the document header.
3. Defining Steps for Validation: The presenter defines the steps for validation. Three steps are used for validation: prerequisite, check, and message. The prerequisite is checked first, and then the system applies the check according to the validation rule.
4. Defining Prerequisite: The presenter defines the prerequisite that the company code should be equal to ‘HA01’. If this prerequisite is met, the system will apply the validation rule.
5. Defining Check: The presenter defines the check that the document date should be equal to the posting date.
6. Defining Message: The presenter defines the message that will be issued if the check fails. He defines a custom message ‘Why are you posting this document?’.
7. Activating the Validation Rule: The presenter activates the validation rule for his company code. He chooses to issue an error message, which will restrict the user from posting the document if the check fails.
8. Testing the Validation Rule: The presenter tests the validation rule by trying to post a document with different document and posting dates. The system issues the error message and restricts the document from being posted.

In summary, the video explains how to create a validation rule, define the steps for validation, and test the validation rule in SAP S/4 HANA Financial Accounting.