Would someone please explain exactly what it is
A timeline is basically a sequence of events about what transpired between the WS and the AP about which the BS was kept in the dark. The level of the sexual and the emotional details is something the BS should decide. Dates of the events, how many times did they meet, how many times did they have sex, positions, where did it happen, how long were they together each time, how did they plan it.. I even needed the smallest details (tmi - like who took off whose clothes, etc.). You decide the level of detail since you cannot 'not know' something that you 'know'.
how it is helpful to R?
It depends completely on the BS. Not sure how is it helpful to R. But, it should help you decide whether to R or not. It gives you an idea of your reality and the monster you are living with. You will know what kind of a person your WS really is and would you want to R with the WS? Or if it is a deal-breaker..
Is this something the WS would do for the BS?
Yes. A remorseful spouse should ideally do this for the BS if the BS wants it. In fact, the WS is the only person who can do this. The WS was there. It is a part of your reality which you were not a part of. You need to know and the WS is obliged to do so if they want to R. My WW never wanted to give me a timeline. I forced her to do it. I even put my inputs with my recollection of events, my own mails and chats and FB posts. I was in investigative mode for nearly 2 months making sense of my whole life.
Please note that I have seen on the Wayward forum and with my WW also, this is probably the hardest part for a WS. Takes a long time to get their head out of their asses. And then they feel angry when they look back at it. Aubrie (one of the WSs who has come a long way) wrote a timeline nearly a year and half after D-Day and she had a whole thread running as to how difficult it is/was. I heard the same things from WW. Maybe someone has the link for that thread.
Also, your WS might not give you the whole truth at once. They call it trickle truth here. My WW gave me 3-4 false timelines before she came clean. I knew she was lying. I kept on probing. I kept on asking her. She finally came clean and told me everything. If you get a timeline, verify with your own version of events at the same time.
Ask your WS to share any proof (mails, chats, etc.) to prove that he/she is speaking the truth. People here have even gone lengths to talk to the OP to get the timeline since the WS was not giving any details. The WS should ideally be honest and transparent in writing the timeline.
I have been all over this place with this post, but I hope you get the gist of it.BBF-turned-BH: 28 (Me)
WGF-turned-WW: 28 (EmotionalFool)
POS1: a 'friend'? WW believed it was my 'best friend'!
POS2: her senior at work!
Together - 6 years
Married - 1.5 years
Don't know where we are headed..