Hi Anton
If you have a S number issued for certification then you can use it to logon to SMP and access notes as well as for use in training.sap.com and SCN if you want
However, if you are on a customer/your work site you will need an S number issued by them so it's against your installation number if you want access to raising incidents, etc for your systems
I have two S numbers (my personal certification one which I own) and then a second S number that I get for a client site (I might have several depending on how many clients I have)
Regards
Colleen