[forms] What's the difference between "Request Payload" vs "Form Data" as seen in Chrome dev tools Network tab

I have an old web application I have to support (which I did not write).

When I fill out a form and submit then check the "Network" tab in Chrome I see "Request Payload" where I would normally see "Form Data". What is the difference between the two and when would one be sent instead of the other?

Googled this, but didn't really find any info explaining this (just people trying to get javascript apps to send "Form Data" instead of "Request Payload".

In Chrome, request with 'Content-Type:application/json' shows as Request PayedLoad and sends data as json object.

But request with 'Content-Type:application/x-www-form-urlencoded' shows Form Data and sends data as Key:Value Pair, so if you have array of object in one key it flats that key's value:

{ Id: 1, 
name:'john', 
phones:[{title:'home',number:111111,...},
        {title:'office',number:22222,...}]
}

sends

{ Id: 1, 
name:'john', 
phones:[object object]
phones:[object object]
}