Why, I don't believe there is a uniform system in place so much as individual arrangements between authors and proofreaders or betas.
It is, after all, no big deal to find out beforehand what files your partner can read and convert your own accordingly; since you don't need any fancy formatting, even a .txt will do in a pinch. Personally, I use LibreOffice and can convert to whichever is needed. It also allows me the option of tracking changes I make to texts (rendering them in colour and underlined), meaning that when I send a proof-read file to someone, they'll see where I've made annotations or added commas or the like.
Of course, if you don't want to use email, there's always options such as googledocs or dropbox.
As I said, it's up to you and your partner(s).
It is, after all, no big deal to find out beforehand what files your partner can read and convert your own accordingly; since you don't need any fancy formatting, even a .txt will do in a pinch. Personally, I use LibreOffice and can convert to whichever is needed. It also allows me the option of tracking changes I make to texts (rendering them in colour and underlined), meaning that when I send a proof-read file to someone, they'll see where I've made annotations or added commas or the like.
Of course, if you don't want to use email, there's always options such as googledocs or dropbox.
As I said, it's up to you and your partner(s).