I have checked all the elevator that connect at same time SNCB and Stib and they all indicate -1 for sncb and -2 for stib : https://www.mapillary.com/app/?lat=50.8433882&lng=4.3805516&z=19.9&pKey=369076086265429&focus=photo&x=0.4882536740574941&y=0.35384446281284304&zoom=0 

I know now why user put a different level labelling : the technical room inside station SNCB are indicated -2 and STIB -3 but it's not a public indication just for technician , and to complicate more https://www.openstreetmap.org/node/7982305392 that deserve Only SNCB platform 3 indicate -1 ;-/ 