I was looking at the documentation for PartialFunction in this link:
trait PartialFunction[-A, +B] extends (A) ⇒ B
"+" and "-" means covariant and contravariant types respectively. In short, it means that:
PartialFunction[-A1, +B1] <:
PartialFunction[-A2, +B2] only if
A1 :> A2 and
B1 <: B2,
<: is subtyping relationship.
"-" usually applied for input parameters, "+" for output - in C# they even use respective keywords
out. By default (without modifiers)
PartialFunction[A1, B1] has no direct relationship to the
PartialFunction[A2, B2] (in other words, it's invariant).
There is also some restrictions, like covariant("+") type can't be in contravariant position (you can only return it from functions) and vice-versa. This is done to support Liskov Substitution Principle and naturally understandable by "in"/"out" interpretation.