The clue used specific words (such as “is bigger”) to let you know that making a particular type of relational response to these triangles would be reinforced. In RFT, cues like this are called contextual cues.
According to RFT, certain features of the context, such as phrases like “is bigger than,” become associated with certain types of relational responding. With enough training, these contextual cues can start to predict reinforcement for making a particular type of relational response. Contextual cues are what allow humans to arbitrarily apply relational responses. These contextual cues, for example, allowed you to make an arbitrary relational response to these triangles.