IRS Form W-2G Form and Instructions
IRS Form 1042-S Non-Resident US Aliens Witholding and Instructions
Gambling Session Definition by Russ Fox
Instructions for Publication 529
The above link describes the IRS defintion of a gambling session as well as what deductions are allowable. It agrees with Russ Fox's
session definition in his article linked above as well.
Page 12 of the above link to Pub 529 describes the
diary you are required to keep and it's contents.