Without knowing the underlying facts your question really cannot be answered.
However, if I hazard a guess, it means that your aunt will be receiving the royalty payments that went unpaid because the energy company did not know who to pay them to after the previous owners death.
Once your aunt returns the W9, so that income can properly be reported to the IRS, the funds will be paid to your aunt and the payment will continue so long as your aunt retains an interest in the property.
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This sounds very suspicious to me. Did this come completely out of the blue? Someone discovered she owns property she knew nothing about? This really needs to be investigated to make sure it is real because it could very well be a scam designed to obtain your aunt's social security number for illicit use. Remember the old adage, "if it sounds to good to be true, it probably is."
They say you get what you pay for, and this response is free, so take it for what it is worth. This is my opinion based on very limited information. My opinion should not be taken as legal advice. For true advice, we would require a confidential consultation where I would ask you questions and get your complete story. This is a public forum, so remember, nothing here is confidential. Nor am I your attorney. I do not know who you are and you have not hired me to provide any legal service. To do so would require us to meet and sign written retainer agreement. My responses are intended for general information only.
Before providing your aunt's taxpayer identifying information, you should Google the energy company and contact the better business bureau to make sure this is not an identity theft scam. You might also want to ask for documentation showing how she can to own the rights.