The modelo 111 is the quarterly return through which you pay the tax office the IRPF you've withheld from others: employees on payroll or professionals who invoice you with retención. It's filed 1–20 April, July and October, and 1–30 January.
The modelo 111 is the other side of retención de IRPF: the one you file when you're the one withholding from someone. If you have employees on payroll or pay another freelancer who invoices you with retención, you pay that withheld IRPF over with the 111.
It's quarterly, with the same windows as the other returns: 1–20 April, July and October, and 1–30 January. If you have no employees and pay no invoices with retención, you normally don't have to file it.
The 111 enters your life the day you hire someone or work with other professionals who apply retención. That withheld money isn't yours: you hold it for a moment and pay it in for them.
Freelancers who withhold IRPF from others: through employee payroll or invoices from other professionals that include retención. If you don't withhold from anyone, you normally don't have to file it.
Cece takes care of what weighs on your day-to-day (projects, payments, proposals, tax calculations) and surfaces the numbers that matter, so they stop weighing on you and you can get back to what counts.
