I am not quite sure what this form is about (AFTER reading this form's instruction from the irs.gov site).

quote from the 1041-T form, Purpose of the form:

"A trust or, for its final tax year, a decedent’s estate may elect
under section 643(g) to have any part of its estimated tax
payments (but not income tax withheld) treated as made by a
beneficiary or beneficiaries. The fiduciary files Form 1041-T to
make the election. Once made, the election is irrevocable"

does irrevocable mean just for this year or forever?
if I don't plan on making estimate tax payments, do I have to file this form?

if I DO plan on making estimate tax payments, and I didn't file this form.
what will happen when I file my fiduciary tax return and the beneficiary's tax return next year?
will I lose something? I just don't get it.

This is my first year filing a fiduciary return that was created in 2011.
it have a rental income and need to depreciate the rental.
I do expect to have a income in the thousands after deductions and depreciation. and there is no other income.

Fiduciary estimated tax payments, made on behalf of a trust, can be passed to the beneficiaries by making an election on or before the 65th day after the end of the trust's taxable year. To make this election, the trustee must complete and file Form 1041-T. Form 1041-T may not be used to pass income tax withheld to beneficiaries except for backup withholding. In the instructions for Form 1041-T it states "A trust or, for its final tax year, a decedent’s estate may elect under section 643(g) to have any part of its estimated tax payments but not income tax withheld treated as made by a beneficiary or beneficiaries. The fiduciary files Form 1041-T to make the election."