There are two ways to restrict the substucture of a user-defined library in Simulink 7.5 (R2010a)
1) Mask the subsystem and add a parameter. Just masking it is not sufficient. You may choose to neither display nor implement this dummy parameter. For example:
In the library, Optimal.mdl, a dummy parameter 'mu' is added to the LN01 block. If this library is opened in the SIMULINK library browser, you will see that the blocks it contains are not displayed.
Regarding the block description in the SIMULINK browser, the mask description is displayed if available. If not then you can right-click on the block and enter the description in the "Block Properties". In the example above, a Subsystem has been added to the library. The Subsystem has Inports and their description is picked up from the mask description even though there is a block description in the block properties field. The subsystem however, does not have a mask description and therefore picks its description from the "block properties".
2) You may register a customization with an sl_customization.m file. There is information on this located in the documentation under Simulink -> User's Guide -> Customizing Simulink Environment and Printed Models -> Customizing the Simulink User Interface -> Customizing the Library Browser -> Disabling and Hiding Libraries
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