Regarding the W-9 that US taxpayers are required to complete if accepting the Exxon dissent offer: One is only subject to the 28% backup withholding tax if they have previously been notified by the IRS that they are subject to such backup withholding for a prior failure to report all interest and dividend income to the IRS. Signing the W-9 provides a certification that the signer is NOT subject to backup withholding.
Regarding those dissenters who were beneficial shareholders [ie. held their shares through a brokerage, which was the actual registered shareholder], such dissenters must obtain the physical dissenting shares certificates from their brokerage [who will obtain it from the Depository Trust Corporation]. Such dissent certificates must be surrendered to Exxon, along with the signed Settlement Agreement and Release, and the completed Appendix page [for address and identification purposes] and a completed W-9 for for US taxpayers.
For dissenters who held shares registered in their own names, they must also surrender their dissenting shares certificates to Exxon in order to accept the Exxon dissent offer [and also provide a signed Settlement Agreement and Release, a completed Appendix page, and a completed and signed W-9].