NSCC illiquid charges are charges that apply to the trading of low-priced over-the counter (OTC) securities with low volumes.
Open net buy quantity represents the total unsettled share amount per stock at any given time during a 3-day settlement cycle. Open net buy quantity must be less than 5,000,000 shares per stock for your entire firm
Basically, you can't hold a long position of more than 5 million shares in an illiquid OTC stock without facing a fee. You'll still be assessed this fee if you accumulate a long position of this size by breaking your purchase up into multiple transactions.
Open net sell quantity represents the total unsettled share amount per stock at any given time during a 3-day settlement cycle. Open net sell quantity must be less than 10% percent of the 20-day average volume
If you attempt to sell a number of shares greater than 10% of the stock's average volume over the last 20 days, you'll also be assessed a fee.
The first link I included above is just an example, but it makes the important point: you may still be assessed a fee for trading OTC stocks even if your account doesn't meet the criteria because these restrictions are applied at the level of the clearing firm, not the individual client.
This means that if other investors with your broker, or even at another broker that happens to use the same clearing firm, purchase more than 5 million shares in an individual OTC stock at the same time, all of your accounts may face fees, even though individually, you don't exceed the limits.
Technically, these fees are assessed to the clearing firm, not the individual investor, but usually the clearing firm will pass the fees along to the broker (and possibly add other charges as well), and the broker will charge a fee to the individual account(s) that triggered the restriction.
Also, remember that when buying OTC/pink sheet stocks, your ability to buy or sell is also contingent on finding someone else to buy from/sell to. If you purchase 10,000 shares one day and attempt to sell them sometime in the future, but there aren't enough buyers to buy all 10,000 from you, you might not be able to complete your order at the desired price, or even at all.