If you don't have horizontal meteo data measurements, you can use the irradiance measured in the collector plane. This is the reason why you should define the orientation of the solarimeter here.

PVsyst will recalculate the Horizontal Global and Diffuse irradiations,  which are likely to produce this tilted plane irradiation (inverse transposition).

During the data analysis and simulation comparisons, you will be able to use either the original values or a recalculated plane irradiation. At this stage, plane orientation may be slightly modified, but big changes are not advised as the tilted plane irradiation looses some information with respect to horizontal true measurements.