Three kinds of customers will purchase products during a single month: new customers you paid for (e.g., from market and sales efforts), new customers you acquired from referrals, and returning customers. This line (T1) is just the sum of these three numbers, located on lines N1, N2, and R1.
Related Questions:
Why do I have no new (paid) customers in a specific market?
Why do I have no new (referral) customers in a specific market?
Why do I have no customers returning to repurchase in a specific market?