eClass and ETIM are two different standards for product information.
eCl@ss is a cross-industry product data standard for classification and description of products and services emphasizing on being a ISO/IEC compliant industry standard nationally and internationally. The classification guides the eCl@ss standard for product attributes (in eClass called properties) that are needed for a product with a given classification.
ETIM develops and manages a worldwide uniform classification for technical products. This classification guides the ETIM standard for product attributes (in ETIM called features) that are needed for a product with a given classification.
It is worth noticing, that these two standards are much more elaborate than for example the well-known classification system called UNSPSC, as UNSPSC only classifies products, but does not tell which attributes (and with what standards) you need to specify a product in detail.
There is a cooperation between eClass and ETIM which means, that you can map between the two standards. However, it will not usually make sense for one organization to try to use both standards at the same time.
What does make sense is combining the two standards, if you as merchant use one standard and your manufacturer use the other standard. The place to make the combination is within Product Data Lake, the new service for exchanging product information between manufacturers and merchants.