comp:cleft relation is used in cleft sentences for the dependency from the head of the sentence to the head of the complement clause.
Sometimes, there is an ambiguity between a relative clause and a cleft clause. Consider the sentence It is the electric car that I chose.
If someone utters this sentence in a context where they must choose between a gasoline car and an electric car, the sentence is interpreted as a cleft sentence with the following annotation.
But if the sentence is uttered in a context where the speaker is referring to one of several electric cars, the sentence is interpreted as having a relative clause with the following annotation.