The actual value of an instanceís copy of a slot can either be stored with the instance or with the class. The local facet specifies that the value be stored with the instance, and this is the default. The shared facet specifies that the value be stored with the class. If the slot value is locally stored, then each instance can have a separate value for the slot. However, if the slot value is stored with the class, all instances will have the same value for the slot. Anytime the value is changed for a shared slot, it will be changed for all instances with that slot.
A shared slot will always pick up a dynamic default value from a defclass when an instance is created or initialized, but the shared slot will ignore a static default value unless it does not currently have a value. Any changes to a shared slot will cause pattern-matching for rules to be updated for all reactive instances containing that slot.
Example
CLIPS> (clear)
CLIPS>
(defclass A (is-a USER)
(role concrete)
(slot foo (create-accessor write)
(storage shared)
(default 1))
(slot bar (create-accessor write)
(storage shared)
(default-dynamic 2))
(slot woz (create-accessor write)
(storage local)))
CLIPS> (make-instance a of A)
[a]
CLIPS> (send [a] print)
[a] of A
(foo 1)
(bar 2)
(woz nil)
CLIPS> (send [a] put-foo 56)
56
CLIPS> (send [a] put-bar 104)
104
CLIPS> (make-instance b of A)
[b]
CLIPS> (send [b] print)
[b] of A
(foo 56)
(bar 2)
(woz nil)
CLIPS> (send [b] put-foo 34)
34
CLIPS> (send [b] put-woz 68)
68
CLIPS> (send [a] print)
[a] of A
(foo 34)
(bar 2)
(woz nil)
CLIPS> (send [b] print)
[b] of A
(foo 34)
(bar 2)
(woz 68)
CLIPS>