#SelfDoubt

Question

If P1 is reducible to P2,

then what does it exactly mean?

What should we assume  

1. )   P1 is a subset of P2

                            or

       b.)   P2 is a subset of P1 

Please answer within the accordance that 

1. If P1 is decidable then P2 may be decidable or undecidable.
2. If P1 is undecidable then P2 is also Undecidable.
3. If P2 is Undecidable then P1 may be decidable or undecidable.
4. If P2 is Decidable then P1 will be definitly Decidable.