The C form is a tax return for non-residents of the Netherlands who receive income from Dutch sources. This may include salary from a Dutch employer, income from property in the Netherlands, or a pension.
You need a C form if you live outside the Netherlands but receive income from Dutch sources: working remotely for a Dutch company, owning property in NL, receiving a Dutch pension or social benefits.
Non-residents are entitled to certain deductions, especially if they qualify as "kwalificerend buitenlands belastingplichtige" — meaning they receive 90%+ of their income from the Netherlands. In that case, the same deductions as for residents apply.
Determining your tax status, analyzing applicable tax treaties, preparing and filing the C form, and optimizing deductions.