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The Form 4506 is used to obtain a full copy of your income tax return. The Form 4506-T is used to obtain certain information pertaining to your return, such as:
1) Transcript of the tax return,
2) Information for your tax account,
3) Your wage and income information,
4) a record of account (your tax liability vs. tax payments you have made).

So, as you can see, Form 4506 requests a full copy of your return and the Form 4506-T requests one or more elements of the return. By the way, the "T" in 4506-T probably represents the term Transcript.

I got the above answer from IRS website.