It might help to sort out the BS if you consider that COsine stands for complementary sine. In other words it COsine is the sine of 90-the angle.
In an particular force resolving situation, you can use sine or COsine or both.
Starting from the rather pithy response from b9710 we can resolve P parallel to the plane as P as P cos 20 or P sin 70. Similarly to illustrate the point the horizontal component of F is either Fcos 20 or F sin 70.
If you are taking notes off a teacher, they might be inconsistent in whether they use sin or cos in a particular situation from question to question. The thing is you could do all the resolving using cos or all the resolving using sin. Most people use sin for one component and cos for the other with the same angle. You could use sin for both and use (in this case) P cos 20 for parallel to the plane and P cos 70 for perpendicular to the plane. OR you could use P sin 70 for parallel to the plane and P sin 20 for perpendicular to be plane.
In this case, MOST people would use P cos 20 for parallel to the plane and P sin 20 for perpendicular to the plane.