Oracle Database Discussions

I have the doubt with the statement like :

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SQL> alter system set OPTIMIZER_FEATURES_ENABLE=8.1.7 scope=both;
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is going to permanently alter the system or for that particular instance or session. I mean is it done once or everytime in a new session. Also, how to see the currect value of this parameter, I mean how to query it?

I hope, my question is clear.

Please, help in solving the doubt.

regards