When multiple versions of STAAD.Pro are installed on the same system, the most recently installed version is invoked by default, even if the input file was created using an earlier / later version.
Therefore, it is recommended to explicitly specify the required STAAD.Pro version when opening the file, using the “Open With” option like AutoCAD.
This is a standard Windows feature. It retains and manages the application that is used to open a given file extnsion. When a new version of STAAD.Pro is installed, then that is deemed to be the application to use. However, like any windows application the user can instruct Windows which executable to use and as such can point it to a different version of STAAD.Pro installed on their computer. This is not something we can control, it is a Windows function.
When multiple versions of STAAD.Pro are installed on the same system, the application automatically defaults to launching the most recently installed version. This behavior may result in users unintentionally opening a version different from the one required for their project. It is recommended that Bentley consider providing a configurable option or built‑in mechanism that allows users or administrators to explicitly select the preferred default version. Such a feature would greatly enhance usability and help avoid version related inconsistencies.