Extended data (xdata) is created by AutoLISP. If an entity contains extended data, it follows the entity's normal definition data. The group codes 1000 through 1071 describe extended data. The following is an example of an entity containing extended data in DXF format.  AutoCAD defines a special  block  *Uxx  for visually repsentation of CSG enities

Let's take CSG box enity apart and put it in order:

(
    (-1 . <Entity name: 600000aa>)                               Enitiy name it changes and does stay the same
    (0 . "INSERT")                        0     - Entity Type  Where "INSERT" means the enity is a block
    (8 . "0")                                         8     - Name of Layer
    (5 . "17")                                       5     -  Unique Name of Enity never to change.
    (2 . "*U2")                                                              2     -  Not  a  Unique Name   "*U"  means that the block is a CSG enity.
    (10 0.0 0.0 0.0)                                                     10    -   x 0.0 , y  0.0 , z 0.0  The base point or insert point
    (41 . 1.0)
    (42 . 1.0)
    (50 . 0.0)
    (43 . 1.0)
    (70 . 0)
    (71 . 0)
    (44 . 0.0)
    (45 . 0.0)
    (210 0.0 0.0 1.0)                             210   - This is default unless extruded.

    (-3
        ("AME_SOL"
            (1002 . "{")                                                    1002  -Extended data control string can be either "{"or "}" These braces enable applications to organize their data by subdividing the data into lists. The left brace begins a list, and the right brace terminates the most recent list. Lists can be nested.
             (1070 . 1)
             (1071 . 1.58946e+006)
             (1070 . 1536)
             (1070 . 1)
             (1010 4.0 5.0 6.0)
             (1005 . "0")
             (1005 . "18")
             (1005 . "0")
             (1040 . 0.0)
             (1040 . 1.0)
             (1070 . 0)
             (1000 . "MILD_STEEL")
             (1000 . "0")
              (1002 . "}")                                                    1002  -Extended data control string can be either "{"or "}"
         )
    ("AVE_INVOKE"
        (1000 . "SOLAVEMESH")
    )
)
)

(

  (-1 . Entity name: 37b0648)     -1 - AutoCad Entity Name (AutoCAD does this Automatically)

  (0 ."LINE")                        0 - Entity Type

  (5 . "89")                       69 - Handle name (if handles are turned on)

  (8 . "STR")                             8 - Name of Layer

  (10 9.0 7.9375 0.0)               10 - Start Point of Line

  (11 11.1213 10.0588 0.0)          11 - End Point of Line    

  (67 . 0)                         Don't worry about this

  (100 . "AcDbLine")               Don't worry about this SubClass Marker.

  (100 . "AcDbEntity")             Don't worry about this

  (210 0.0 0.0 1.0)                Don't worry about this.  This is default unless extruded.

)

Extended data for  cone

Command: co
Baseplane/Elliptical/<Center point> <0,0,0>:

Diameter/<Radius>: .5

Apex/<Height>: 2
Phase I - Boundary evaluation begins.
1 of 3 of Phase I in process.2 of 3 of Phase I in process.3 of 3 of Phase I in process.Phase II - Tessellation computation begins.
1 of 2 of Phase II in process.2 of 2 of Phase II in process.Updating the Advanced Modeling Extension database.


Command: hfg

Select object: It is a Cone!!
(
    ("cone"
        (0.0 -2.44921e-016 -2.0)
        (0.5 0.5 2.0)
      )
)

Command: !xdata
(
    (1002 . "{")                                           1002  -Extended data control string can be either "{"or "}" These braces enable applications to organize their data by subdividing the data into lists. The left brace begins a list, and the right brace terminates the most recent list. Lists can be nested.
    (1070 . 1)                                             1070  A 16-bit integer (signed or unsigned)
    (1071 . 1.69152e+006)                        1071 A 32-bit signed (long) integer
    (1070 . 1537)                                       1070  A 16-bit integer (signed or unsigned) name of primitive  cone = 1537
    (1070 . 1)
    (1010 0.5 0.5 2.0)
    (1005 . "0")
    (1005 . "31")
    (1005 . "0")
    (1040 . 0.0)
    (1040 . 1.0)
    (1070 . 0)
    (1000 . "MILD_STEEL")
    (1000 .  "0")
    (1002 . "}")                                           1002  -Extended data control string can be either "{"or "}"
)