You can use either.
Although if you want to use both, you need to make sure you start the sensor with the right one. Unfortunately, I can't remember which that is but if you search for "Libre" on this forum (use the Search field in the top right), it has been mentioned a few times.
Provided the link has been made to Diasend (or whatever your DN is using - ask them how this is done), the clinic can see your readings.
They cannot tell how many times you scan provided you scan at least every 8 hours (the sensor stores 8 hours of data so if you do not scan that often, it will lose some data)
The LibreLink app does not do any calibration - it relies on factor calibration.
You can enter comments if you want.
There are other apps (such as Glimp) which allow calibration but these are not linked to any DSN/diabetes clinic
There have been a lot of threads about Libre on this forum.
I strong recommend reading through them as many people have the wrong expectation on things such as accuracy, how to get the sensor to stay in place and any ways to pimp your sensor.
These questions (and more) have been answered multiple times on the forum.
By the way, I am surprised you have been prescribed Libre without training which should explain this and how to get the most out of the sensor.
This is not a criticism of you but of your diabetes team. Libre can be very powerful but only if you know how to interpret the readings.