UGOpen中基本概念

在UGOpen中有很多基本的概念
例如Piece Part  ， Occurrence  ，Instance ，Prototype
他们都是什么关系啊，UG虽然给了说明但是好像还不是很清楚，那位大虾帮忙解释一下
  
Terminology and Concepts:
  
The following are terms used:
  
Tag               - is being used for the old term EID (Entity IDentifier) and
                    for the new term object identifier. A Tag identifies an
                    object in the Unigraphics database.  The objects are not
                    limited to geometrical objects, but can include parts,
                    instances, and occurrences.
  
Piece Part        - This is the lowest level part in an assembly; it contains
                    geometry and features that represent the physical object,
                    but no components.
  
Occurrence        - When several parts are brought together into an
                    assembly, the parts and their objects are not copied
                    into the assembly part. Rather, the parts are loaded into
                    memory, and part occurrences of those parts are put into
                    the assembly part.
  
Object Occurrence - For each object in the piece part, an object occurrence of
                    that object is made in the assembly part.
  
Instance          - This is the term used to indicate the placement of a
                    component part within an assembly part. For each Instance,
                    a Part Occurrence is displayed in the assembly part.
  
Multi-level       - Assemblies can be multi-leveled.  For example, an
                    automobile can consist of the body and two instances of an
                    axle assembly part, which itself consists of an axle and
                    two instances of a wheel assembly part, which itself
                    consists of .... etc. In these cases, each Instance can
                    have more than one associated Part Occurrence.
  
Component Part    - This is the usage of a part within an assembly, at a
                    particular location and orientation. A component may be a
                    sub-assembly consisting of other, lower-level components.
                    Each component in an assembly contains only a link to its
                    master geometry.  When you modify the geometry of one
                    component, all other components in the session using the
                    same master update automatically to reflect the change.
  
                    Old (V9) style components are stored in the Database as
                    Type 63, Subtype 0. New (V10) style components are listed
                    as Type 63, Subtype 1.
  
Prototype         - This is the master geometry to which an occurrence is
                    linked.  In the case of a part occurrence, it is the piece
                    part containing that master geometry.
  
Reference Set     - This is a named collection of geometry from a Unigraphics
                    part that may be used to simplify the representation of
                    the component part in higher level, large, complex
                    assemblies.  Whenever the name of a reference set is passed
                    in to a routine, if the string is NULL or blank, the whole
                    part is used.  If the string "Empty" is used, the empty
                    reference set is used.
  
Displayed Part    - This is the part that is being viewed in the graphics
                    window. It can either be a piece part or an assembly.
                    The displayed part must be fully loaded.  A partially
                    loaded part can be made the displayed part by opening it.
  
Work Part         - This is the part whose geometry or assembly structure is
                    being modified. The Work Part can be the same as the
                    Displayed Part or any component of the Displayed Part if
                    it is an assembly.  The work part may be partially loaded,
                    but must be a member of the assembly which is the displayed
                    part.  Only objects of the work part may be edited.
                    Object occurrences can not be edited, but they may be
                    queried to find their data.

找本UG的书应该没问题吧