DENIED NUMBERS is a list of numbers you can't call, typically 900 numbers.
ALLOWED NUMBERS is a list of numbers users can call even if they normally can't call long distance, such as 800, 888, 877 etc.
ARS is automatic route selection: the system looks at the number dialed and decides how to send it...trunks to use, digits to add, delete or echo.
If you're not using an OUTGOING button on phones, 8 is the feature code usually used to place an outside call (people used to use 9 for this). Depending how your system is programmed it might pick ARS or might pick a trunk group. You can see what this does by looking at SYSTEM/DEVICES/PHONES/XXX/ASSOCIATED EXTENSIONS/OUTGOING...you should see either 92000 (ARS) or a trunk group number.