The short answer is not much. No matter what the Unit is called you still deduct a certain number of units for using a room. It just depends on how many Units you deduct for using the room. Deducting a session for every minute they tan on a 100 Session package is the same as deducting 1 minute for every minute they tan on a 100 Minute package
The long answer is more complicated.
Mostly it is how the salon positions itself for marketing and the sale of their packages. SalonTouch uses 3 different Unit types: Sessions, Minutes, and Points. This gives the salon flexibility and can leverage that for their business model.
Session packages are those where a client can tan for any length of time in a bed. Session package often cost more and are designed around individuals with a solid base tan who will want to tan for the maximum time every time. They feel they get more out of it even when they are not on a monthly package.
These would be deducted using Flat sessions for the Room Use section. Every time they use the room, it will deduct 1 flat session no matter how much time they tan.
Minutes packages are those where a certain number of minutes are deducted for each minute tanned in the bed. This is normally a 1 to 1 deduction where 1 minute of tan time deducts 1 minute from the package. Minutes packages are geared to those who are either building their tan or the "Frugel" client who want to conserve their Minutes over a longer time.
These would be deducted using Per Minute sessions. The units being deducted would be based on the number of minutes tanned.
Points packages are similar to minutes, but some clients feel that they get more value out of a Point than a Minute. You can use this to bring in clients who do not want a session package, but feel the big-box salons charge too many minutes for tanning with upgrades. The client may feel 3 Points per minute for the Level 3 is perfectly fine, but 3 Minutes per minute for the Level 3 is not.
It all comes down to how you want to market your salon to your client base. The session-based tanning charges based on the tan itself, and the minutes and points-based tanning charges based on the time that they are tanning.