First of all, you have to satisfy whatever is required by the Govt Regulations in the area that you are drilling.
Secondly, you have to meet the Company wide guidelines of ENI.
I don't know what they are, but would hazard a guess that they are based on either Oil and Gas UK abandonment guidelines (Well Decommissioning Guidelines Issue 6 of June 2018) or Norsok D-1010 rev 4 as noted by Peter.
In most cases, satisfying those two requirements will pretty much dictate where you need to have barriers to ensure zonal isolation and therefore where you HAVE to cut the casing (as opposed to NEED to cut - which may be different).
Your program should then be based on cutting the casing at a depth which is as shallow as possible while still enabling you to meet all the criteria.
There may be other factors which could also be taken into account, for example, if you need to recover the casing for use on another Well.
Lastly, how you complete the final section of P&A depends on whether it is Land, JU, Platform or Semi/Drillship.
All the best!