Incorrect spelling, explanation: referal is incorrect as in refer the last syllable is stressed, so r should be double when adding a suffix (like in prefer or confer). Therefore referral is the correct form. In different words which also end with -er but don’t have stress on the last syllable, we do not double the r (like consider). Here however we do, since there is a stress on -er, so referral is the correct form.

Correct spelling, explanation: we only double the r if the stress is on that syllable (for example refer, prefer). Therefore we double r in referral. This is also why in other words which end with -er we don’t double r with suffixes – for example, consider the stress is on sid, not on er. This explains why referral is the correct form and not referal.

Definition of referral:
noun, the act of referring or something that is referred
Please show a referral number when making an appointment with the doctor.
A good referral system in a company is an important factor in its activity.