What is the difference between FIFO and standard SNS?

Excuse me, could you please clarify the difference between FIFO and the standard SNS approach in the context of cryptocurrency or finance? I understand FIFO stands for First-In, First-Out, but I'm not entirely sure how it differs from the standard SNS method. Could you explain the key differences and their implications, perhaps with some real-world examples or scenarios where one might be preferred over the other? Thank you in advance for your insight.