Foreign Transactions & Entities

Good Night, I’m confused as to what is the distinguishing feature between a monetary asset/liability and a non-monetary asset/liability. I know there are common examples. However, I really want to understand the concept so I won’t have to regurgitate a list. Additionally, let’s say an entity has a foreign bank account and it needs to be retranslated at year end using the closing rate. In the exam, we are given the closing rate but in practice how do we determine the closing rate. I’m wondering about this because usually the bank has a buy and sell rate at year end and I’m confused if the closing rate we should use is the average of the buying and selling rate as at year end or is it either the buy rate or the sell rate? Thank you.