From IRCTC site:
If found travelling without any proper authority to travel, passenger
will have to pay fare with penalty between original boarding point to
changed boarding point. Boarding station can be changed before 24
hours of the scheduled departure of train.Boarding point change is not
allowed for current booking ticket
From Make My Trip:
While you may choose a boarding point as any station on the route of
the train, you would have to pay the fare as applicable for
Reservation From – Reservation To stations. For example, if you are
booking a ticket from Mumbai Central to New Delhi and choose the
boarding point as Kota Jn, then you can board the train only at Kota
Jn and not between Mumbai Central and Kota Jn. You would however have
to pay the fare for Mumbai Central to New Delhi
Before you board at B, if the TTE comes for his rounds, and he sees that your seat is empty, if he wishes, he may wait till the next point before allotting your seat to a waitlist or RAC person. He has all rights to do this or he could fine you.
From IRCTC again regarding change of boarding points
The boarding point change is available , Only for the train tickets
that booked online not for the reservation counter tickets. But the
online tickets can be changed through 'Electronic Reservation Slip'
before 24 hours scheduled departure of train as per extant Railway
rules in reservation counters.
You could instead book from A to C and until 24 hours before the departure of the train you can change your boarding point to B with no issues at all. I think you're booking from A to C due to better availability of seats quota from A than from B. If this is the case, its always better to book ticket from A to C and then change A to B. That way you don't have to pay the fine for not travelling between A to B as per rules