An A contact is someone who has already referred others to you or is highly likely to do so. B contacts may refer if reminded. C contacts may become a referral source at some point in the future. D contacts are unlikely to refer, so it makes sense to contact them only if you've exhausted all your A, B and C options that week.

Not sure how to determine A versus B contacts? Keeping track of their transaction histories gives you a snapshot of their lifetime value to you in terms of transactions and referrals. More on that here. (link to recording transactions)