Site experience means you were employed as an Engineer (corrosion or materials or other) at a site where you participated in either management of day to day operation/maintenance activities or you were a process engineering specialist. This means actively involved with handling or coordinating day to day engineering processes and dealing with corrosion or material issues.
My two cents,
Site experience refers to direct contact with actual engineering events related to your job, as mentioned above 'field evaluation and troubleshooting experience'. This includes direct exposure to event analysis under senior supervision and involvement in solvind various events over a longer period of time (as opposed to one or few isolated events).
Cheers,
gr2vessels