"transaction" means goods and money exchanging hands - i.e. an instance of a dealing. "volume" is a loose word for a measure of scale (dimension, weight, magnitude, frequency, number etc). So the two are not directly related, but the important attributes of transactions are the number of instances and the amount of money involved. If someone lists transactions and volume, most likely the former is "number of transactions" and the latter the amount of money involved in the transactions.