CIF - Game Info - Opening
eco contains the ECO code computed by the application. This code is in general not reliable, because the ECO computation has some limitations. Different applications may compute different ECO codes.
Element usereco contains the ECO code given by the user. Normally the application is giving this ECO code a higher precedence than the computed code.
classification contains an assigned opening name, probably combined with a variation name.
A note about Scid: Scid is using extended ECO codes like "A43j". It is not allowed to export this extended code, it has to be shortened, but it is appropriate to use an additional private tag for this extended code.