Okay I know this is a question I am going to get. Now I am a bit of a rules lawyer and that usually my answer to questions like this. "It's in the rules." However, perhaps a more in depth answer would help soothe the natives. So the question I am going to get is why am I using the availability enforcement? Usually we just make characters and buy what they want with no limits. Any thoughts?
Availability is quite literal, it's how easily available something is. Lower availability rating means it's much easier to get through the black/gray market or through contacts. Let's say you wanted to get Intel's latest prototype CPU. Now, as I'm sure you're aware there are only a few hundred of these in the world. How would you get one if you only have the normal retail channels (white market) available to you? The answer is, you can't.
Now, if you have a contact, let's say a research scientist who works at Intel, I would say that the chances of you getting that prototype are much better. It would still be a bit far-fetched, but it's at least possible now. Let's say you have a different contact who happens to the lead quality tester for the prototype line. It'd be much easier for him to say "this chip is bad" and
steal it from the company to sell to you. So in essence, different types of contacts with different levels gives you better or worse chances at getting what you want.
At character generation you can buy any legal gear that's under the availability limit from stores through certified cred sticks. Anything approaching the higher availability numbers represents stuff that's harder to find... but you can find it eventually if you look hard enough. The hard availability limit, by default is Rating 6 gear and availability 12, is meant so that starting characters don't have better gear at the beginning. You can explain this any way you like.
Personally I like to allow characters to get what they want, but anything outside of the starting limits needs to be explained.
-Availability level is default, Rating 6 and Availability 12
-Restricted items can be registered to a SIN or fake SIN, but not required. If it's not registered, how did the character obtain this item?
-Forbidden items not allowed without an explanation. How did the character obtain military grade gear?
In essence, anything outside of the default can be explained either by back story or through contacts linked in that story. If a character grew up with a guy who ended up working pretty high up in Ares as an explanation of why he has a Stoner-Ares M202 MMG (availability 12F), he'd better have that friend on his contacts list with a pretty high loyalty rating. Usually I limit characters to only one and
maybe two items outside of the chargen availability limit. If the characters can't have cool stuff at chargen, it gives them some goals to work towards. If a person were to write an excellent back story and have his or her character interconnected with all their contacts then I may allow more items. At that point though I might actually start rolling like you would in game to see if they can actually start with that item.