Basically, they're trying to make sure you are able to take full advantage of their match. They're doing the right thing by giving you the information in advance.

How exactly does your bonus work? Do you get the payment in a check, or does it get paid directly to your 401(k)? My company deposits annual Christmas bonuses into their 401(k) accounts, rather than an extra paycheck. I believe this is relatively common.

This is a good thing, for tax reasons. If they pay an extra paycheck, they would either have to withhold a bunch of tax, or you could potentially end up owing a bunch of money on the next year's 1040. I worked as a tax preparer many years ago. I worked with several customers that got nice Christmas bonuses, then immediately blew it all on gifts/food/clothes/gadgets, and then got furious when they found out they owed a bunch of tax on it (since none was withheld). I had to be the bearer of bad news, because their money to pay back the IRS had long since been spent.

Anyway, in either case, you'll have an uneven distribution of 401(k) matching throughout the year if you use the same % for the whole year. This is because your income isn't evenly distributed (since you're getting an extra check very early in the year).

If you're not expecting to max out your 401(k) at $18,000 this year, then you have no need to worry about this at all. If you do think you'll max it out, you'll need to make sure you hit the limit on your last paycheck (and not any sooner - because then you would be forfeiting some of the company's match). To do this, there's a chance you might need to lower your deferral %, which is why they're sending this e-mail.

Does that all make sense?