In SNA, control information preceding a request/response unit (RU), that specifies the type of RU (request unit or response unit) and contains control information associated with that RU.; 在系統(tǒng)網(wǎng)絡(luò)體系結(jié)構(gòu)(SNA)中，位于請(qǐng)求應(yīng)答單元(RU)之前的控制信息，它描述請(qǐng)求單元或應(yīng)答單元的類型，并包含與RU有關(guān)的控制信息。