Form fields lock if PDF signed?

I'm familiar with digital signatures or "certificate signatures" as they are called in DC. But I had come to think that the "Sign" tool under "Fill & Sign" simply added a superficial graphic of a signature that had no bearing on document permissions or editability. That's certainly the rule if the PDF has no security restrictions. But I just handled a PDF form generated by an outside party that will lock (flatten?) if a "Sign" signature is added and the PDF saved. The PDF form is locked, but fields are listed in permissions as editable- and still list as editable even after the fields are no longer accessible. And just to be clear, there was no prepared signature field- you could put the "sign" graphic in the footer next to the page number and the locking/flatteningbehavior would be the same.

There are two ways to look a this, and I'd appreciate help from either angle:

- Form prep security settings that enable this kind of behavior

- A description of "Sign" tool functions explaining when it amounts to more than a signature stamp.

Thanks.