Hello Jonáš Hubáček, trunk classification is assigned to main roads with a motorway-like layout in Kenya. The Kitale-Cherangani Road, part of the C51 route, appears to branch out to towns and villages. And when compared to the overall road network, trunk would make the roads stand out relative to similar ones that a) extend across the country or b) connect between major cities and towns. Hopefully that answers your question.