Bogleheads:Sandbox

 is a type of deferred 401(k) subaccount, with different rules from traditional and Roth 401(k) accounts. They are typically used in strategies to rollover money to Roth IRAs far in excess of normal contribution limits.

Advantages

 * Subject to much higher contribution limits than traditional 401(k)s (up to $52,000 in 2014)
 * Some 401(k)s allow for in-service distributions, allowing employees to roll over funds during employment without a triggering event.
 * Protection from creditors under ERISA.

Disadvantages

 * Death, for monies waiting for rollover, the inheritor is left with a non-deductible IRA with very complicated paperwork.
 * Gains before rollover are taxed as ordinary income.
 * Losses before rollover, "basis" could be lost if you don't leave a few pennies in the account.
 * Management Risk: the rules can be changed without warning.
 * Management Fee/Check writing fees may be high enough to eat into savings.
 * Trustee Risk: it's my understanding they will be sending you two 1099-R's every year, for the rollover and for the gains, pray they calculate it correctly.
 * Legislative Risk: hopefully you could complete the rollover before the benefits were ended.
 * Additional paperwork: many accountants aren't aware of the documentation requirements.
 * Company fails nondiscrimination tests, resulting in a refund (See Highly compensated employee).

After-Tax 401(k) limits
Traditional and Roth accounts share a employee contribution limit up to $17,500 per person (for the 2014 tax year), rising to $18,000 in the 2015 tax year. However, Section 415(c)(1)(A) limits total contributions to defined contribution plans to $52,000 in 2014. The limit for an after-tax 401(k) is the difference between the amount already contributed by the employer and employee, and the Section 415 limit.