This is taken from Phil's article on the Wiki:
http://trac.secdev.org/scapy/wiki/LengthFields

Peter had written a very nice tutorial on his web site
http://thesprawl.org/memdump/doku.php?id=tools:scapy
and kindly allowed me to add it to our docs under the
CC BY-NC-SA-2.5 license.

RandDraw*: random draw from a set without replacement.
           Guarantees that a name is not drawn twice
           before all elements have been drawn.
Rand*:     random draw from a set with replacement.
RandSing*: singular values from a set, drawn at random
           with replacement.

ex: ICMP code field

* * *

* * *

Typical use is to register many different kinds of a packet at their
declaration.

A deprecation warning will be issued before complete removal.

class Test(Packet):
    fields_desc = [ IntField("test1",0),
                    SNAP,
                    IntField("test2",1) ]


>>> ls(SNAP)
OUI        : X3BytesField         = (0)
code       : XShortEnumField      = (0)
>>> ls(Test)
test1      : IntField             = (0)
OUI        : X3BytesField         = (0)
code       : XShortEnumField      = (0)
test2      : IntField             = (1)

(Thanks to rmkml)