Non-compete clauses are common agreements signed by freelancers and come in various forms. They can be presented as a standalone agreement or as a section called out in a larger document. In either case, it's important you understand what you're agreeing to when you sign. Things are more complicated if your client additionally requires you work with a 3rd party vendor they set up to handle contractors. In addition to signing agreements with your direct client, you will have to sign agreements with their 3rd party vendor. The 3rd party vendor agreements are the ones you need to read extra carefully because they also contain non-compete clauses different from your direct client's.
They key info to look for in the vendor's agreements states that any future work you do with your direct client after the contract ends must be processed through the 3rd party vendor for a period of time set in the vendor contract. The time listed is typically 12-18 months.
Let's say you take a year-long contract with Company A and they require you to work through their vendor Company B. The agreements you sign with Company B state that if Company A asks you back anytime within 12 months after your contract ends, you must go through Company B to do so, or face breach of contract legal consequences.
So, 5 months after your contract with Company A ends, they call you back for new work and want you to use a different vendor, Company X. You are technically still under contract with Company B. It's only been 3 months since your last contract ended and Company B has you for 12 months. If you choose to take the contract and work with Company X, you run the risk of being sued.
The world of freelance work involves shifting agreements and responsibilities. Being a self-employed person means it's your responsibility alone to understand what you're agreeing to whenever you choose to work with a new client. Read carefully and ask questions if you don't understand something in a contract.