If direct trading in a pair is absent or volumes are low, the rate is calculated through a currency with sufficient trading volume. The US dollar typically serves as an intermediary: the euro to sum rate is obtained by comparing the euro to dollar rate and the dollar to sum rate.
This leads to an important consequence: the cross rate depends on two quotes simultaneously. The ratio of sum to euro can change not because something happened in the domestic market, but because the euro to dollar rate changed in the global market.
For sum pairs with euro, ruble, yuan, tenge, and other currencies, this means their dynamics only partially reflect internal processes. Comparing the sum to ruble rate without considering how the ruble moved against the dollar leads to incorrect conclusions.
A practical note on exchange: actual bank rates account not only for the calculated ratio but also for the spread, which in cross operations is typically wider since the conversion effectively goes through two transactions.