What do you have going on that you need to attach something to support retained earnings that isn't already on the return?
ProSeries own review, tells you a statement should be attached:
"Unappropriated retained earnings on Schedule M-2, l,ine 8 is greater than $250,000. You must be able to show a business purpose for accumulations in excess of $250,000, or you may be subject to an excess accumulations tax. If there is a businss purpose for the excess, attach an explanation to the return."
Here is from the IRS:
Accumulated Earnings Tax
A corporation can accumulate its earnings for a possible expansion or other bona fide business reasons. However, if a corporation allows earnings to accumulate beyond the reasonable needs of the business, it may be subject to an accumulated earnings tax of 20%. If the accumulated earnings tax applies, interest applies to the tax from the date the corporate return was originally due, without extensions.
To determine if the corporation is subject to this tax, first treat an accumulation of $250,000 or less generally as within the reasonable needs of most businesses. Treat an accumulation of $150,000 or less as within the reasonable needs of a business whose principal function is performing services in the fields of accounting, actuarial science, architecture, consulting, engineering, health (including veterinary services), law, and the performing arts.
In determining if the corporation has accumulated earnings and profits beyond its reasonable needs, value the listed and readily marketable securities owned by the corporation and purchased with its earnings and profits at net liquidation value, not at cost.
Reasonable needs of the business include the following.
Specific, definite, and feasible plans for use of the earnings accumulation in the business.
The amount necessary to redeem the corporation's stock included in a deceased shareholder's gross estate, if the amount does not exceed the reasonably anticipated total estate and inheritance taxes and funeral and administration expenses incurred by the shareholder's estate.
The absence of a bona fide business reason for a corporation's accumulated earnings may be indicated by many different circumstances, such as a lack of regular distributions to its shareholders or withdrawals by the shareholders classified as personal loans. However, actual moves to expand the business generally qualify as a bona fide use of the accumulations.
The fact that a corporation has an unreasonable accumulation of earnings is sufficient to establish liability for the accumulated earnings tax unless the corporation can show the earnings were not accumulated to allow its individual shareholders to avoid income tax.
While it doesn't state a requirement to attach a statement (still looking, so it may not be a REQUIREMENT), but I have to wonder, how would the IRS know you have a business purpose for the excess if you didn't tell them you do, in the form of a statement.