To provide some background, I'm an MS student (CS - machine learning) looking to apply for PhD programs. I was looking for letter writers, and struggled to find the third.
I have the option of asking a postdoc that I've worked with or a professor who has explicitly stated that he "doesn't know me well enough to write me a detailed and strong letter, but if you have no one else I'd be glad to write one."
My question is, what does it mean to "co-sign" a letter? Many people have advised me that if a postdoc writes my letter then it would be a good idea to have our advisor co-sign it. As far as I know, universities send out requests to submit letters electronically and writers submit the letters online. My dad is also a professor (albeit not CS) and has said he hasn't heard about this ever since letters went online. What would this mean in today's context? Do some fields still request mail-in letters?