added some notes to add context

Author: gregsdennis

specs/jsonschema-core.md

@@ -413,7 +413,7 @@ they do not know how to process or explicitly choose not to process.
 ## Keyword Behaviors {#keyword-behaviors}
 
 JSON Schema keywords may exhibit one or more behaviors. This specification
-defines three such behaviors:
+defines three such behaviors[^7]:
 
 - Assertions validate that an instance satisfies constraints, producing a
   boolean result: `true` if the constraints are satisfied; `false` otherwise.
@@ -422,6 +422,12 @@ defines three such behaviors:
 - Applicators apply subschemas to parts of the instance and combine their
   results.
 
+[^7]: This specification also defines several operational directive keywords,
+such as `$id` and `$schema`. As such, these keywords do not exhibit these
+behaviors. However, it is recommended that extensions avoid defining additional
+directive keywords as they could interfere with schema processing and produce
+unexpected or undesirable results.
+
 Extension keywords SHOULD be defined using these behaviors, keeping in mind that
 annotations in particular are extremely flexible. Complex behavior is usually
 better delegated to applications on the basis of annotation data than
@@ -584,8 +590,14 @@ identifying such referenced schemas is defined by the keyword.
 Applicator keywords also behave as assertions by defining how subschema or
 referenced schema boolean [assertion](#assertions) results are modified and/or
 combined to produce the boolean result of the applicator. Applicators may apply
-any boolean logic operation to the assertion results of subschemas, but MUST NOT
-introduce new assertion conditions of their own.
+any boolean logic operation to the assertion results of subschemas, but SHOULD
+NOT introduce new assertion conditions of their own.[^2]
+
+[^2]: It is recommended that keywords have a single resposibility. An example of
+this in action is the separation between `properties`, which verifies object
+property values have the right data _if_ they exist, and `required`, which
+verifies that object properties exist. Separating these concerns allows for more
+reusable components.
 
 [Annotation](#annotations) results from subschemas are preserved in accordance
 with {{collect}} so that applications can decide how to interpret multiple