The offshore account is used for purchasing contracts on Crime.net, offshore paydays, ascending the first five levels of Infamy (requiring the player to spend $200,000,000 per ascension) and to unlock the third, fourth and fifth skill profiles costing $1,000,000, $10,000,000, and $20,000,000 respectively.
At the end of a successful heist, 80% of the player's earnings are deposited into the offshore account, while the last 20% becomes "spendable" cash. Prior to Update #11, the split was 95/5.
The offshore account is securely laundered, and deposited into untraceable accounts in nations that exercise banking anonymity and discretion. The total amount is visible on the screen shown in the safehouse vault. Meanwhile your spending cash is stacked in the form of physical US Federal Reserve bills in the vault itself.
As of update 95, Offshore Payday has been made permanent to the game. The "Offshore Payday" icon appears on the left side of the Crime.net screen. This lets you place a wager using large amounts of offshore money to get certain Card rewards. The cost of the offshore payday depends on the type and the number of "secured" (100% certain) cards that are going to be awarded to the player after this phase. The number of "Secured" cards can be 0, which is the cheapest, up to a maximum of 3, which is the most expensive.