We choose "Chat and Instant Messaging" category in "URL Category":

To block certain URLs we need to choose URL Reputation from 6 to 10.

The following are restrictions for Flexible NetFlow:
+ Traditional NetFlow (TNF) accounting is not supported.
+ Flexible NetFlow v5 export format is not supported, only NetFlow v9 export format is supported.
+ Both ingress and egress NetFlow accounting is supported.
+ Microflow policing feature shares the NetFlow hardware resource with FNF.
+ Only one flow monitor per interface and per direction is supported.
Reference:
consolidated_guide/b_consolidated_3850_3se_cg_chapter_011010.html
When configuring NetFlow, follow these guidelines and restrictions:
+ Except in PFC3A mode, NetFlow supports bridged IP traffic. PFC3A mode does not support NetFlow bridged IP traffic.
+ NetFlow supports multicast IP traffic.
The Flexible NetFlow - MPLS Egress NetFlow feature allows you to capture IP flow information for packets that arrive on a router as Multiprotocol Label Switching (MPLS) packets and are transmitted as IP packets. This feature allows you to capture the MPLS VPN IP flows that are traveling through the service provider backbone from one site of a VPN to another site of the same VPN

Explanation