As I said in the other topic, some other doubts are arising.
Looking at the Ships’ Sheets (and in the Ships’description too) there are some “Available” Modules under the Installed ones (i.e. the Cerberus has the Long Range Scanner installed, and the Nexus Link available for the Comms Modules); what is the role of the available ones? Are they suggested because they are particularly useful to that ship?
Sorry I missed something in the book (again).
I struggled with this one as well and don’t think you missed anything. 
After talking with others, I believe the intent is that Available modules are available from the start (should they choose to go that route) and are more readily available for Advancements. Other modules may require more work to access such as a job, narrative retrofitting, etc.
That’s my take on it, anyway!
After digging around various fora, I arrived at the same conclusion as @Shakes. If a module is “Available”, it is easy to get and doesn’t require any special justification on the part of the crew to obtain.
Whether or not this is actually beneficial depends a lot on the GM’s style. For example, it’s a nice benefit if the GM is very strict about how the crew can come by acquiring and installing modules. However, if the GM is very permissive and doesn’t require much justification about how the crew acquires modules then it doesn’t really matter.
In my game, I basically ignore availability. I ask players to give some small explanation for how they came across the module, but that’s mostly for establishing the fiction.
Here’s an official answer from John LeBoeuf-Little
What you’re talking about are ‘available modules’ and their purpose is to make clear what systems are easily found and installed for ships of that type. While nearly any module can be fit into any ship (any cable rerouting is easily accomplished, no matter the system or ship) some modules are easier to source.
Mechanically, this means that, when you advance your crew and gain upgrades, you may freely spend them on these available modules. Getting Afterburners installed into the Stardancer is a trivial affair. We can handle that off-screen. But getting any sort of weapon installed on the ship will take a bit of explaining - a downtime action, a job, or often free play with some of your contacts.
As a GM, how strict you are about the availability of modules is up to you, and says things about the world. I like to use fortune rolls using the Tech or Crime/Syndicate ratings of the planet in question. Regardless, available modules should always be easily available - that’s what that term means.