What is fortigate IPS set status default ?

When it is set to default, the action for that signature is the one shown in the Action column.

@smartgate : Are you reffering for the signature configuration. 

If referring to signature config -- then status default sets the action of the signature to default whatever defined in signature definition. Or you can change the signature from default to custom block. 

For more clarity , you might to share screenshot or config that you referring to... Even there is a default IPS Security Profile as well. 

It has been confirmed that the default IPS status setting operates according to the default status of each signature in the IPS signature category. Here's a question. Which signatures are registered with a status of disable? By what criteria is this selected, and is there a reason why this disabled signature does not seem to work?

Disabled signatures are based on signature definitions update or manalluy disabled. 

To find out all disabled signatures , you can apply a column for status and filter with disabled. You can see below screenshot on how to find out disabled signatures. 

Yes, I know. What I'm curious about is that if you set the status to default and select a signature with the default status of disable like in this picture, this signature won't work, so I'm curious why there is a separate disable signature.