Hello Asham Deep Singh Saini;
I’m no expert on OPCua, from what I know, those are created to comply the OPC10000-018 role base security, you can check on the link for more information
Regarding your second question, I have personally never used the opcUa python library, but if what you say about the AS 4.5 is true, then it’s likely that some security parameters on OPCUa configuration can be the cause, since in AS4.5 quite a few of the security parameters are set to true by default that on AS6 are disabled.
You may want to try to enable diferents security policies…For instance in AS4.5 by default No security is allowed in conversation,and AS6 is disabled by default, etc I’d start from here to check. You can also start by enabling all of them and disabled those not needed by your application, that depends on you.