Since 2000, the FCC has permitted "modular certification," under which it approves a transmitter module for use in multiple host devices. That option originated with a unilateral public notice from the FCC, rather than a rulemaking in which the public participated.
Today the FCC released a "Second Report and Order" that codifies and extends the 2000 public notice. (The First Report and Order in this docket made other changes to the Part 15 rules without addressing modular certification.)
The new order carries over most elements of the 2000 public notice almost intact, with two significant changes.
First, devices incorporating modular transmitters may now display the FCC ID in electronic form. Electronic FCC ID display has previously been allowed only in software-defined radios.
Second, the FCC adopted new rules to govern "split modular transmitters." In these, the radio front end and controlling firmware are separate, possibly located in different components. Manufacturers must ensure that only components that have been certified together are capable of operating together.
An escape clause allows a manufacturer to bypass any of the modular requirements if it can persuade the FCC that the device will nonetheless comply in all of the end products in which it is used.
A copy of the order is at this link.