Hi, glad you found it useful!
Each interface can only have a single ACL applied to it in any direction, one inbound and/or one outbound.
The direction you will choose to apply the access list is in large dependent on the topology itself and way traffic flows through it,
as well as the how and what traffic you, the admin, decide which traffic to block or permit and where in said topology.
A few rules of thumb:
Standard ACL's are best applied as close to the destination as possible. The reason being they only match on the source IP address thus blocking any traffic that matches a certain entry in the ACL and that may result in blocking of entire networks.
Extended ACL's are able to match on a plethora of properties (i.e. source / destination IP addresses, source / dest. port & L3 Protocols). Because of that level of granularity available to us we can afford (and best practice suggests that we do) to place the ACL as close to the source as possible. Imagine being routed throughout a large network only to arrive at the destination and eventually be blocked, a waste of resources.
As for the direction application of the ACL, this again ties back to the way the topology is but largely this is how this works;
Inbound ACLs are check when traffic tries to enter an interface (whether a virtual or physical one) and before the packet has been processed by the forwarding plane. Traffic that matches a permit statement and manages to get through an inbound ACL (assuming there is one) gets processed and routed to the appropriate egress interface it is at this point (post routing) that the traffic is checked against an outbound ACL, right before it exists the interface (again assuming one is applied).
You can see how this can take up unnecessary resources (processing and routing of the packet only to have it eventually dropped), which is why it is sometimes preferable to match (and drop) on an inbound ACL, but there are situations and constraints that will prevent you from doing so (remember one ACL per-direction per-interface).
I hope this answered your question.
Cheers;
