The Exodus and the 1^st and 2^nd Succession wars completely ruined the Inner Sphere’s tech base. Mechs are now built the equivalent of 2^nd Gen tech – this is the so-called “IS tech level”. It is the technological equivalent of pre-to-early Star League.

Don’t make the mistake of thinking it isn’t advanced: It includes the ability to build gigantic walking tanks powered by fusion engines, armed with potent weapons and protected by nearly impervious armor, each more than a match for a platoon of tanks. Some tech fields lag a little behind, such as electronics, but they still advanced enough to guide said mech – and jump starships safely between the stars.

There is some variety within this tech level as well. The ability to build light mech frames and fusion engines still exist, for example. Not quite Endo-Steel/XL engine level, but still pretty advanced. Other examples include weapons with slightly better (or just different) stats from the baseline models. Many of the most advanced 2^nd gen techs can only be created using highly automated factories from a long lost era. This kind of tech isn’t lostech per se, but it’s rare, expensive and not suitable for general use.

Lostech starts where 2^nd Gen ends and 3^rd begins. This is the mech technology from the mid-to-late Star League. Some of it is still in circulation though, testament to the skill of those who designed it and the durability of its construction. Much of it is classified as relics of a bygone era. The prices of such things are easily 10 times or more that of comparable 2^nd Gen tech. If they can be had at all. Classic examples include ER energy weapons, pulse lasers, Artemis guidance systems, Ultra autocannons, double heat sinks, and super-complex computer systems.

Then there are the 4^th Gen stuff, which was cutting edge even before the end of the Star League. The best examples of users are the “Royal” mechs, which were built using the best of 3^rd Gen with a sprinkling of 4^th Gen. The line is kind of blurred, and it can be hard to tell the generations apart (in GBT terms they are tagged with “experimental”). 

Below all of this this is 1^st Gen tech, sometimes called Periphery level tech, which is theoretically sufficient to build mechs, but only mechs with more primitive components. Could be full weight fusion engines that take up as much space as an XL engine. Or primitive armor that provide less health per ton.

Note that mechs don’t have to have a uniform tech level. A mech could be built in the IS using common 2^nd Gen tech, but after years in the Periphery it has only 1^st Gen armor – but a lostech pulse laser of the 3^rd. A SLDF “Royal” mech would likewise be made entirely from lostech of the 3^rd Gen, but could also have some systems not generally available to the rest of the Star League.

Some lostech still exist. Some mechs may contain original components, or at least some of them. Other mechs have access just to a single weapon or other lostech system that’s been fitted later.

Lostech is either preserved, scavenged or it is produced (in very limited numbers):

Preserved lostech are those bits that have lived on since before the lostech era (most people think ‘Star League’, but the fact is that it wasn’t until well into the 2^nd SW that the true Dark Age began, so parts aren’t usually that old). If preserved tech has been used all the while, chances are it’s been repaired and rebuilt time and time again – even lostech doesn’t last forever without wear and tear.

Scavenged lostech is stuff from the lost era that’s been recovered. Maybe from a battlefield, maybe from an old storage facility, or from a mech that’s finally given in. Unlike preserved lostech it hasn’t been in continuous use, so it closer to the original. It will likely still need an overhaul before it can be used – stuff that lay around naturally decays. The holy grail of scavenging is finding a lost SLDF cache with mothballed mechs, but truth be told less lostech is scavenged for each passing year.

Produced lostech is made at one of the very rare robotic factories still in operation. Given the cost and complexity of these processes, most factories that could theoretically build lostech have been turned over to producing common mech parts.

Regardless of origin, lostech is expensive, if it can be had at all. If available, it typically costs 10 times or more that of a non-lostech part. Maintenance complexity (difficulty and time) go up, as do maintenance costs. For these reasons fielding a lostech battlemech force isn’t just a stupid expensive investment, it’s also going to be a constant drain on resources. If you get a 20-30% increase in efficiency, but a 100% increase in operating costs, why not simply have more standard mechs?