Postnups can cover a broad range of areas and are customized to every couple's situation.
Postnups can include how assets like financial accounts and property will be split. If you own a business, with or without your spouse, postnups can outline how this business will be dealt with in the event of a separation. You cannot change your personal liability to a creditor, but you can address how the debt will be paid, between you and your spouse, in a postnup. While postnups typically address the division of assets and debts at the time of separation, you can also address on-financial matters, like what tax filing status you will use or how separate property can be created during the marriage.
You can include all of these or just one. It is up to you, your spouse, and your lawyers as to what to include, given your goals and intended outcomes.
In North Carolina, the only areas that cannot be addressed in a postnup are spousal support, alimony, child support, and custody.