From a mechanical perspective, you can install plasterboard with the long edge either horizontal or vertically. What is important is the boards are fixed to framing components at the correct centres.

With systems that are required to have a fire resistance and are installed to the latest fire regulations and standards, in order to be in line with the EXAP associated with that system type, the board orientation is important, and can only be installed as per orientation in the tested system. 

Siniat’s general recommendation is to install all boards and board rips vertically to internal partitions as this is how they are tested by default.