Oracle Database Discussions

In v$parameter does the column value of the following columns mean that...

ISSES_MODIFIABLE FALSE means can not change by alter session

ISSYS_MODIFIABLE FALSE means can not change by alter database

ISINSTANCE_MODIFIABLE FALSE means can not change by alter system

If it is that then confusion about.....

SYS@ORCL>alter system set audit_trail=OS scope=spfile;

System altered.

where as audit_trail is .....

SYS@ORCL>select name,value,ISSYS_MODIFIABLE,ISINSTANCE_MODIFIABLE
2 from v$parameter
3 where name like 'audit%';

NAME VALUE ISSYS_MOD ISINS
------------------------------ ------------------------------ --------- -----
audit_sys_operations FALSE FALSE FALSE
audit_trail NONE FALSE FALSE

Please clear my confusion

Regards