The standard conversions are those pre-defined conversions that can occur as part of a user-defined conversion.
The following implicit conversions are classified as standard implicit conversions:
Implicit numeric conversions
Implicit nullable conversions
Implicit reference conversions
Implicit constant expression conversions
Implicit conversions involving type parameters
The standard implicit conversions specifically exclude user-defined implicit conversions.
The standard explicit conversions are all standard implicit conversions plus the subset of the explicit conversions for which an opposite standard implicit conversion exists. In other words, if a standard implicit conversion exists from a type A to a type B, then a standard explicit conversion exists from type A to type B and from type B to type A.