|“|| With the founding of the United Mars Foreign Legion in SC 0212 came the need for an inexpensive and reliable mobile frame that could be easily built of local parts and modified to local conditions. Based on and expanding the ST-03 standard, ST-07 is now used by almost all UMFL companies. Chubs can be found running on any power system, from power cells to internal combustion engines. Outfitted with sensors and communication gear and a wide assortment of weapons, most Legionnaires will swear by their Chub, though none ever call it elegant.
Its extraordinary reliability coupled with its “repair with local parts” design philosophy also makes stolen Chubs popular with Free Colony cells trying to obtain dedicated military hardware.
–Mobile Frame Zero: Rapid Attack Rulebook , pg. 170
The Chub's ability to be built from local parts grants the UMFL considerable cost savings over importing frames through the transit gates to a colony. Chubs have become known for being durable and reliable, but also slow and heavy.
The Chub's versatile design means it's not uncommon for any given Chub to only share a few common parts with any other given ST-07.
Main Source: Mobile Frame Zero: Rapid Attack Rulebook