@dhanslip you can define your own custom type of platform and it's value of domain
Goto Admin menu and select Manage Project properties -> Lab Resource properties
Click on Installed software, add your own type of custom platform there. Then in child page define possible values of this environment and click save
Now Click on Testplan Environment Types on same page and click on Installed software. you can see here just added custom platform. Now Select and and in the context menu select "Set Type". Accept default in the dialog and click ok. Now again save project properties.
You are all set. Now you can add just added custom platform in your test plan's test environment section and generate environment as you have been doing for default platforms.
Hope it helps.

-|- Pramod

Hi David,
If you are familiar with RQM, Test cases are planned for execution by creating Testcase Execution Record (TER)
A TER describes what testcase to execute but it also tells for which environment to run it against, which iteration of testing to run it and who is the owner for execution.
TER is a unique combination of (Test plan, Test Case, Test Environment, Iteration)
Typically you will
1) First define the test environment by creating combination of test environments (OS, DB, Browser etc..)in Test Environment section of Test plan. e.g. Firefox_Windows7
2) You will create TER for the selected test cases, this is the time you select the environment where you want to run your test case. For manual testing it is the responsibility of Tester to execute test on same environment as specified in TER. For automation Testing, RQM can run Automation on matching machine after little configuration. See http://www.ibm.com/developerworks/rational/library/automate-tests-environment/index.html for more info.

The process of creating TER is called execution planning where you defined which test case to be executed and where besides other info.

To answer your question, suppose you are assigned 25 TER and you want to search for all TER which are to be executed on environment say Firefox_Windows7. you can goto Browse Testcase Execution Record view and in the the Test Environment filter you can enter Firefox_Windows7 and run query besides other filter like owner="your id"
this will list to all TER assigned to you for a specific environment.
There is another way say where you can search based on keyword says "Windows". Type the keyword in the filter box at top right corner. It should display all TER which contains Windows in name for any of the Testcase name, Test Environment Name, etc..

Hope it helps
-|- Pramod