How to fill out the column with conditioning text

[Shaopingfu]

How can input certain text in column "Text" - 

1) if currently the "Text" column is empty or with null, and column - Offset vendor is filled with 8-digit number - input "Payment" + "Offset vendor description" but limit to not greater than 20 characters. 

2) if currently the "Text" column is empty or with null, and column - Offset vendor is not filled with 8-digit number - input "Payment"

3) If currently the "Text" column is not empty, and column - keep existing text and highlight the cell with Yellow

[Zok]

Hello again

[Zok]

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