What is a difference between loan and credit in the law system? Let's say US law. What does it mean to grant a loan, use a loan or to have a credit? What regulations describe both credit and loan?
The terms are used in inconsistent and overlapping ways. Their meaning has to be determined from context. Words do not have universal meanings in all contexts in the law.
A loan generally refers to a delivery of something (often money) with a legally binding expectation that it will be returned with some additional compensation to the lender, later on. To "use a loan" would mean actually receiving a loan from someone.
A "grant of a loan" usually means agreeing to make a loan in the near future, rather than actually carrying out the loan at that time.
Credit is a broader term with multiple senses. When a purchase is made "on credit", the sale is accompanied by a loan to the buyer to assist the buyer in making the purchase.
But, the term a person's "credit" can also refer to an ability to borrow money, rather than to money that has already been loaned.
A third sense of the word "credit" is a technical accounting sense of the word. In double entry accounting, a credit is an event that increases a person's assets or reduces their liabilities (in contrast to a "debit" which does the reverse). A transaction that is a credit to one party in double entry accounting is usually a debit with respect to the other party, or is part of an offsetting debt-credit pair of impacts on the person's books in a single event.