Non-grafted linear frames, known as powered exoskeletons, exos, or, rarely, sidekicks, are designed to be an intermediary step between grafted linear frames and the full-scale powered ACPA suits like the ones defined in Maximum Metal. The linear frame outlined in CP2020, p. 92 is "grafted onto your body, while its systems are directly neurolinked to your muscles and bones," i.e. you can't take it off, hence the 2D6 to 3D6 HC.
This is all well and good, but what if you just want to wear a frame instead of living in one?
Well, you can go to CP2020, p. 67, and "chip into the suit as if it were any cyberbike or vehicle, taking a -2 REF penalty to do so." At this point, you are wearing an exo, NOT a linear frame. It may seem like a trivial distinction, but it's an important one.
Exos follow the same basic rules as the standard LF's, with one exception: you can get out of them. These are advanced versions of the "clumsy and hard to control" exoskeletons described on CP2020, p. 92, and are in most respects identical to the LF in appearance and function. The operator dons the sidekick in modular pieces, and jacks into it with a Machine/Tech link or V-link (CP2020, p. 82). ACPA suits can be built around them, but cannot be built around the CP2020 rulebook-defined linear frame (as above). Unlike standard linear frames, powered exoskeletons apply a -2 penalty to REF and MA, since they are not a direct part of the user's body (-1 for advanced -- read expensive -- models, and nice GM's may elect to nuke the penalty entirely for skilled users, like those with an Operate Heavy Machinery/Pilot Exo/Pilot ACPA score of 8+). The frames are not sealed, and one cannot swim or run very fast in them. Specialized exos are equipped with modular attachements for loading cargo and so forth. Otherwise, all stats are identical, with the exception of Humanity Cost, which is negated.
Linear frames cannot exceed STR 16. The human body won't stand up to it. Period. If it's grafted on to you, STR 16 is the ceiling. What, you want a higher strength? Go full 'borg, ya loser. Put on a friggin' ACPA suit. Get a life.
Advanced exos can go up to STR 52; at this point, you're about ready to hang a chassis on it and call it Assisted Personal Combat Armor (ACPA). When the wearer is hit, basic exos and linear frames (STR 12-16) take damage on a 2 in 10 chance; advanced exos (STR 17-52) take it on a 3 in 10 chance. For damage purposes, the exo's SDP is equal to its STR. Note that this is for bare exos only; ACPA suits SDP is half their STR, presumably because of internalized and more sensitive controllers (being more complex, they break down easier -- it's simpler to wreck a cellular phone than an old-fashioned dumbbell receiver). A damaged frame will continue to function at full abilities until it fails a breakdown roll; this is equal to the percentage it has been damaged (a STR 34 exo, with SDP 17 and 5 points of damage on it, has a 30% chance of failure each turn). The percentage chance goes up 10% per each hit the frame takes after it's reached 50% SDP (when that same frame hit 9 points of damage, for instance, it's at 52% breakdown rate; when it hits 10 points, it's at 62%). The majority of exos are made for utility purposes, not combat. They provide little protection to the wearer (logically, if the exo's hit, the wearer isn't), and will break down quickly if damaged.
Instead of the clumsy and rather silly Reality Interface Systems, ACPA uses only Aperture-Based, Wideband Aperture-Based, or Full-HUD Wideband. Likewise, Reflex Control has gone much too far; it can only consist of Manual (equivalent to Basic Control, where you just wear the damn thing, kinda like Ripley in Aliens), Trodes with a V-Link (equivalent to Advanced), or Jacked (equivalent to Low Boost, requiring a Neuralware Processor and a V-Link, without the +1 bonus mentioned in Maximum Metal). ACPA slows Kereznikov Boosts or Sandestivan Speedware down to a REF 10 max, and the fact that the interface is designed for normal folks who don't normally exceed REF 10. Why build in mega-reflex capacity when an average joe can't take advantage of it?. Likewise, Adrenal Boosts will make you vibrate like a monkey in the ACPA frame, but you won't move the suit any faster. Expensive custom-made suits could be designed for higher REF users, for a hypothetical cost of 1000eb per point of REF.
The term sidekick is derived from John Varley's story "Blue Champagne." However, in CP2020 terminology, both Varley's Golden Gypsy and William Gibson's polycarbonate exoskeleton from "The Winter Market" are linear frames, since both authors indicate that the devices are directly and permanently linked to the user's nervous system (for long-term treatment of quadreplegia, though they may be removed in emergencies, if desired). I use the term sidekick in a different sense from Varley's definition; I stole it for an alternate slang word to describe a non-grafted linear frame, since NGLF is kinda clumsy. CP2020 players can avoid confusion by calling all permanent attachements linear frames, and all removable attachments sidekicks or exos, depending on function; sidekicks are generally smaller and more fragile, while ACPA suits can be built around exos.