Here's a guide to the different options depending on the type of subscription:
SNCB Subscribers
A MOBIB card containing your SNCB subscription
SNCB + STIB Subscribers
A single MOBIB card containing your SNCB subscription and your STIB subscription
SNCB + TEC Subscribers
A single MOBIB card containing your SNCB subscription and your TEC subscription
SNCB + De Lijn Subscribers
A MOBIB card containing your SNCB subscription and your De Lijn subscription
SNCB + STIB + TEC Subscribers
A single MOBIB card containing your SNCB subscription, your STIB subscription and your TEC subscription
SNCB + STIB + De Lijn Subscribers
A single MOBIB card containing your SNCB subscription, your STIB subscription and your De Lijn subscription
SNCB + De Lijn + TEC Subscribers
A MOBIB card containing your SNCB subscription, your TEC subscription and your De Lijn subscription
SNCB + STIB + De Lijn + TEC Subscribers (SNCB + Maximum 2 other operators)
A single MOBIB card containing your SNCB subscription, your STIB subscription, your TEC subscription and your De Lijn subscription