Of course for the full story you probably have to decipher the relevant chapters of their rulebook (https://www.iextrading.com/docs/Investors Exchange Rule Book.pdf)
Searched for "peg" in that rule book and came up with this section:
Rule 11.190. Orders and Modifiers
(b) Order Parameters
(10) Discretionary Peg Order
(C) Is not eligible for routing pursuant to IEX Rule 11.230(b) and (c)(2).

