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

1. Substitution Rules: The video explains that substitution rules are used to replace the values entered and re-entered according to the prerequisite rules defined in the substitution rules.
2. Creating a Substitution Rule: The presenter demonstrates how to create a substitution rule in the system. He creates a substitution rule for his company code ‘HA01’ and applies the substitution at the line item level.
3. Defining Steps for Substitution: The presenter defines the steps for substitution. Two steps are used for substitution: prerequisite and substitution case. The prerequisite is checked first, and then the system applies the substitution according to the substitution rule.
4. Defining Prerequisite: The presenter defines the prerequisite that the general ledger account should be equal to the salary expense account. If this prerequisite is met, the system will apply the substitution rule.
5. Defining Substitution Case: The presenter defines the substitution case that the text field should be replaced with the constant value ‘Salary expense is posting’.
6. Activating the Substitution Rule: The presenter activates the substitution rule for his company code.
7. Testing the Substitution Rule: The presenter tests the substitution rule by posting a financial document with the salary expense account. The system applies the substitution rule and replaces the text field with ‘Salary expense is posting’.

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