Symantec Backup Exc 15 license

Good day,

I have Symnatec Backup Exec 15 and licenses had installed on Windows Server 2012 with information as below:

A) BACKUP EXEC SERVER ED WIN 1 SERVER ONPREMISE STANDARD PERPETUAL LICENSE        (Entitled Qty : 1)

B) BACKUP EXEC OPT LIBRARY EXPANSION WIN 1 DEVICE ONPREMISE STANDARD PERPETUAL LICENSE    (Entitled Qty : 1)
    
C) BACKUP EXEC AGENT FOR VMWARE AND HYPER-V WIN 1 HOST SERVER ONPREMISE STANDARD PERPETUAL LICENSE    (Entitled Qty : 2)

Service Exp : Jan  20, 2017
SC Service Type    ESSENTIAL

I have some questions, anyone can help me clearly explain :

1) what does "onpremise" mean and function in symantec license ?

2) If I have 3 virtual machines in Hyper-V and I must to buy how many  BACKUP EXEC AGENT Licenses  for them ?

3) I can upgrade new version of Symantec if service license is expired ?

Thank much so much.