A non-solicitation restraint stops you from actively going after the customers, suppliers, or staff of the business you just sold, without stopping you from working in the industry altogether.
Technically, it prevents the seller from soliciting customers, suppliers, or employees of the target business for a defined period after completion, and it is generally easier to enforce than a full non-compete because it targets specific relationships rather than competitive activity in general.
The buyer wants a broad, long-running non-solicitation covering every customer and employee the business has ever had. The seller wants it limited to relationships that genuinely existed at the point of sale, for a period that reflects how quickly those relationships would naturally move on regardless of the restraint.
How “customer” is defined does most of the practical work here. A precise definition, such as named customers or customers actually served in a defined period before completion, is far more defensible than an open-ended reference to anyone who has ever bought from the business.
What you as a seller should push back on: a customer definition broad enough to capture relationships that never actually involved you personally, restraint periods that run meaningfully longer than the corresponding non-compete, and a solicitation standard broad enough to capture ordinary contact that was not actually initiated by you.