Sometimes it's impossible to fill in all fields in deal/lead forms at once because you just don't have the required information yet.
Make particular fields Required starting with particular stages and fill in fields as you get the needed information.
How it works
For example, let's create a Rejection reason field and make it Required for the Deal lost stage.
Open a deal form > click Create field > String.
Enter the Field name and activate the Required at stage option.
Select Deal lost stage only.
Done! Let's try to change the deal status to Deal lost.
Bitrix24 will ask you to enter the rejection reason.