sine qua non

I thin Sine Qua non relates to Law of Torts.  so the definitions in Law of Torts or ask your tutor.

The usage, " It's a must " , used to be regarded as grammatically erroneous.    Sine qua non used to be the preferred substitute for must.

Grammarians bowed to the public and no more regard it as a grammatical error but have given the status, informal. 

Now some extraneous information to illustrate the influence of the public.

1.  I'm taller than him.     Erroneous previously.     Not now.    I'm taller than he.     Appropriate earlier.

2. It' become absolutely right to use due to instead of owing to.