Primarily, the banker-customer relationship is that of debtor and creditor—who is which depending on whether the customer's account is in credit or is overdrawn.

【译文】银行与储户主要是债务人和债权人关系。究竟谁是债务人谁是债权人要看储户是有结余还是透支。

【讲解】that替代relationship，以避免重复。who is which depending on…overdrawn乃独立主格结构，进一步说明破折号之前的句子。who is which是现在分词短语depending on…overdrawn的逻辑主语。which代表debtor或creditor，意为“谁是债务人，谁是债权人”。whether…is overdrawn作on的宾语。

【单词和短语】

credit 是多义词，可为名词或动词。

n. 信用，荣誉，贷款，赞扬
Start by shoring up your credit. 先支持你的信用。
Mary's painting would do credit to a real artist. 玛丽的画给一个真正的艺术家增添了荣誉。
Any users of credit may overextend themselves. 每个贷款使用者都有可能作出不自量力的事。
Our employees decided to set up a credit fund. 我们的职工决定要设立一笔贷款基金。
I give them full credit for their work. 我高度赞扬了他们的工作。

v. 归功于，信任
Thank you, but in all truth I feel the credit should go to Ted. 谢谢，但是说实话我感觉应该归功于泰德。
His credit is good, you can trust him. 他很守信用，你可以信任他。
Paul has credit with Mr. Walter. 保罗受到瓦特先生的信任。

overdrawn：透支的（if you or your account is overdrawn, you have spent more than you had in your account and so you own the bank money），

例如：
interest charged on overdrawn accounts对透支账户所收的利息。
overdrawn的动词形式为overdrawn。