There is usually a little LED screen next to where you insert your card that will have information scrolling by as you play, like "Hello BlueSkadoo! You have 500 points! You earned 43 points this session!"
Those are session credits and they are strictly for POINTPlay where 1,000 session credits = $10 in free slot play. Used to be you earned 1 session credit for each $1.50 in coin through on slots (and either $3 or $4.50 on video poker), not sure if its the same rate or not... it seems to be.
That little LED screen doesn't show your tier credits. To see those you have to either log in to the MLife website, MLife app or be playing on a few machines that can actually let you log in to view your MLife tier status (almost all the machines at Aria
let you do this, at other MGM casinos some of them will have only a couple of machines in the whole casino that let you see your tier credits).
The session credits are a holdover from the old Players Club program which is why there isn't any mention on the MLife website - they mention "POINTPlay" but don't describe how its earned.
Session credits are also where much of the controversy over which machines are "special" and which ones aren't - most of the misinformed think that session credits are tier credits, which they are not. So when they go to check a machine to see if its a "special" one or not by how its awarding tier credits they end up actually looking at how its awarding session credits, which are much slower than how they award tier credits.
Anyhow... in the end the important thing is that session points are a counter to track how much free slot play you have earned.