The AssetId property is a user writable alphanumeric character sequence uniquely identifying a component. The ID is provided by the vendor, integrator or user of the device. It contains typically an identifier in a branch, use case or user specific naming scheme.
This could be for example a reference to an electric scheme. For electric schemes typically EN 81346-2 is used.
An use case could be to build up a location oriented view in a spare part management client software. It enables to identify parts with the same article number which is not possible if this entry is not used. This property is defined by ComponentType defined in OPC UA DI.