Add note about meta-fields

Author: magicmark

spec/Section 3 -- Type System.md

@@ -2336,6 +2336,34 @@ DirectiveArgumentCoordinate : @ Name ( Name : )
 5. Return the argument of {directive} named {directiveArgumentName} if it
    exists.
 
+**Resolving Meta-fields**
+
+Resolving the _schema coordinate_ of a meta-field is undefined behaviour, since
+these fields may be implemented in such a way that makes it impossible to return
+a unique value specific to that instance of the meta-field.
+
+Therefore, it is left up to the implementation to decide whether or not to
+return a value for a meta-field. If a value is returned, it must meet the
+requirement that it is unique to that _schema coordinate_.
+
+**Result Coercion and Serialization**
+
+A GraphQL service, when preparing a field of a given scalar type, must uphold
+the contract the scalar type describes, either by coercing the value or
+producing an _execution error_ if a value cannot be coerced or if coercion may
+result in data loss.
+
+A GraphQL service may decide to allow coercing different internal types to the
+expected return type. For example when coercing a field of type {Int} a boolean
+{true} value may produce {1} or a string value {"123"} may be parsed as base-10
+{123}. However if internal type coercion cannot be reasonably performed without
+losing information, then it must raise an _execution error_.
+
+Since this coercion behavior is not observable to clients of the GraphQL
+service, the precise rules of coercion are left to the implementation. The only
+requirement is that the service must yield values which adhere to the expected
+Scalar type.
+
 **Examples**
 
 | Element Kind       | _Schema Coordinate_               | _Schema Element_                                                      |