What is the right time to provide Offer Letter & Appointment Letter?

I want to know that offer letter we can give at the time of documentation or not or we can send it in mail.
Can we send the salary bifurcation on mail before joinings or we can provide that with Appointment Letter only.

It's sound HR practice if the offer letter provide key info. like the name of post, salary, other facilities, perks, JD/JT, probation period etc. This should ofcourse precede verification & BGC. Offer can be emailed followed by hard copy posted. Appointment order will contain almost same info with addl.info. like date of joining, reporting and other CTC details and other terms & conditions. Appointment order/letter should necessarily be served in person either while joining or immediately thereafter which should acknowledged & accepted.