To understand the demand for the "991" link, one must understand the software's evolution. Diagbox versions are notoriously fragmented. For years, version 7.01 through 7.57 were the stable standards for older vehicles. However, newer Peugeot and Citroën models (those switching to the EMP2 platform around 2013/2014) require newer software architecture.
Diagbox is a unified software interface that combines two older PSA systems (PP2000 for Peugeot and Lexia for Citroën). Version (often written as 991) is a specific build that supports vehicles manufactured up to approximately 2021-2022. diagbox 991 full espanol link