It is compulsory for drivers to carry a certificate (see documents attached), but only in cases where the driver has to leave the vehicle or return to it. For example, if the driver has only to transit the regions of Navarra and Aragón, or go to the centre where he/she has to load/unload the goods, a certificate is not necessary.
This regulation only applies in the regions of Navarra and Aragón.
Source: Gobierno Foral de Navarra and Gobierno de Aragón
It is also compulsory for drivers to carry a certificate at all times in the region of La Rioja.
Source: CETM Spain / Gobierno de La Rioja