I have done connection design within the STAAD.Pro. If I transfer the model (.std) file to another machine, I do not see the connection design data anymore. Why?

Files .cut, .rcc and .rsd are created in the same folder where the .std file is located when one designs connections in the Connection Design workflow inside STAAD.Pro. If one wants to open the connection design data in another machine, these 3 files with extensions .cut, .rcc and .rsd have to be transferred together with the .std file.

Alternatively, a STAAD.Pro archive can be created, where all the analysis files will be added automatically (including .cut, .rcc and .rsd). The process is explained in this Wiki: Creating an archive of the analysis files