IR35 is the dreaded tax code that forbids contractors from appearing to be "disguised employees".
There's no published fixed set of rules as to what is and isn't inside or outside IR35 but the guidance I've seen seems to me mostly centred on control and direction of labour.
The advice I've had from my accountants is that it's much easier to demonstrate that you're outside IR35 if you charge for deliverables and rather than time.
If you're just a bum on a seat, doing exactly what you're told, then you're indistinguishable from an employee for tax purposes.
As such you can be liable to be taxed twice retrospectively without receiving any benefits of employment such as sick pay, pension, holiday or expenses. That can happen years down the line even when your accounts were fully signed off and paid up.