This will be a dump of various bits and pieces of miscellaneous decking knowledge.
Alternative representation and additional information is also available at the houserules article.
1. Canonically the Matrix requires a subscription to use. Because you're a fly-by-night shadowrunner who refuses to pay the corps their nuyen, not that you have a SIN to pay with, *any* intrusion into the Matrix is considered a hostile one. This means as soon as you step foot into it the tally (security/alert) level begins to climb.
2. Exits themselves (SANs) can be encrypted and there are several bugs on how it is documented.
So, the room is not the SAN, the exit is. An access-encrypted room can prevent movement in, forcing you to decrypt it blind.