This is what I found in the Electronic Devices book by Stephen Fleeman.
During the development of the V-I curves (and on manufacturers' data sheets), it is customary to apply the transistor convention. The transistor convention has been defined in Fig. 4-10(a).
Currents that flow into the transistor are called positive. Conversely, currents that flow out of the transistor are called negative. If the actual conventional current directions agree with the transistor convention, they are called positive. If not, they are called negative.