"Under the federal system" 或 "in a federal system" 都可以表达“联邦制度下的”意思，具体使用哪个取决于句子结构和语境。例如：
* Under the federal system, each state has its own constitution and laws.（在联邦制度下，每个州都有自己的宪法和法律。）
* In a federal system, power is divided between the central government and the regional governments.（在联邦制度下，权力在中央政府和地区政府之间进行划分。）