The backbone infrastructure is administered and patrolled by the Grid Overwatch Division, and is theoretically neutral between the megacorps. But the corps didn’t get rich by trusting each other. Where security demands it, it’s not unusual for corporations to run their own private communication lines - for example between a secure, hidden facility and a more public one. This lets the secure facility access the Matrix discreetly without making its location or purpose obvious. This is called dark fibre.
On the local mesh, dark fibre functions like a wormhole. If you can hack the controlling host on one end, you can coerce it to carry your traffic to the other, and suddenly you can “see” devices that could be dozens or even hundreds of kilometers away. Occasionally, wily shadowrunners use this as part of a smash-and-grab, using a forgotten dark fibre link to hack into a distant host that is too physically well-protected to get near in the physical world.
Tags are tiny, passive chips, with a small amount of ROM and minimal processing power. They use the ROM to store a small number of data, which can then be broadcast onto the matrix. They usually have no battery of their own, or only a very small backup battery; instead, they rely on wireless power transmission from nearby Matrix devices.
Tags cannot be hacked, as such, as they lack any processor to hack. They can be erased by a specialist tool that generates a powerful electromagnetic field, but it only has an effective range of a few centimetres, so you need to know where the tag is.
Tags are very common in my campaign, and have numerous uses:
Broadcasting a fixed AR holo - eg a billboard, signage, an animated menu outside a restaurant, or a piece of grafitti.
Local tracking of things - most goods that cost more than a few nuyen have tags included in the packaging. Warehouses and retailers can use these tags to track inventory and ring up your shopping bill.
Global tracking of things - tags can be configured to upload their location to a cloud server whenever they have a working Matrix connection. People use these as locators for any of their stuff they want to keep track of.
Theft prevention - a variation on the tracking idea, most expensive, durable, or legally restricted goods (definitely including vehicles, guns, and ammunition) are infested with multiple tracking tags. If the goods are noticed stolen, they can be swiftly tracked down via the locations being uploaded from the tags.
Sneaky users might have tags on their items that do not broadcast their location all the time, but sit passive and undetectable until certain times or they receive an incoming signal; this makes it very difficult to know you’ve definitely wiped every tag off something you just stole. Items the characters purchase with from the black market LPs have already gone through this.
The local mesh relies on ultra-wide band signals that can barely penetrate walls at the best of times. So it doesn’t take much to block them almost entirely via smartpaints that use nanotech to assemble a crude Faraday cage as it dries. This prevents any decker outside the area from seeing in; icons for devices inside cannot be seen from the outside, and all hacking traffic is blocked.
Smartpaints are quite expensive, so tend to be reserved for only high-security areas within a facility. These are, of course, combined with physical security measures and access controls. Smartpaints are also wildly unpopular with workers, as they are inconveniently cut off from the outside world.
Personas are a special kind of icon used in VR to represent a human user. They are endlessly customisable, by creating custom 3d models and animations and/or by purchasing expensive digital goods from your favourite brands. Most of humanity is happy with a generic persona, perhaps with a small purchased accessory or two. But for some, particularly those who live in the matrix, persona customisation is an important part of how they express themselves. This is particularly common in decker circles.
People who have spent a lot of time or money customising their persona often want to show it off in AR as well as VR. They run special programs on their commlink that do a public broadcast of their persona as an animated ARO. A miniature version may float over their head or ride around on their shoulder. Some people go so far as to animate a life-size version of their persona and have it envelop their meatbod, effectively hiding them within it. This is very difficult to do well; often they will clip through the animated persona, ruining the effect.