With thanks to John VE3WNA from the comments of this question for the reminder, the answer (rationale/guidelines) is here:
Specifically the following:
There are two basic ferrite material groups: (1) Those having a permeability range from 20 to 850 µ are of the Nickel Zinc (NiZn) class (mix 43, 52, 61), and (2) those having initial permeabilities above 850 µ are usually of the Manganese Zinc (MnZn) class (Mix 31, 73, 75).
The NiZn ferrite cores (mix 43, 52, 61) have low permeability, exhibit high volume resistivity, moderate temperature stability and high ‘Q’ factors for the 500 KHz to 100 MHz frequency range. They are well suited for low power, high inductance resonant circuits. Their low permeability factors also make them useful for wide band transformer applications. Nickel-zinc ferrites have a higher resistivity and are used at frequencies from 2 MHz to several hundred megahertz. The exception is common mode inductors where the impedance of NiZn material is recommended from 70 MHz to several hundred GHz.
The MnZn ferrite cores (Mix 31, 73, 75) have high permeabilities above 800 µ, have fairly low volume resistivity and moderate saturation flux density. They offer high ‘Q’ factors for the 1 KHz to 1 MHz frequency range. Cores from this group of materials are widely used for switched mode power conversion transformers operating in the 20 KHz to 100 KHz frequency range. These cores are also very useful for the attenuation of unwanted RF noise signals in the frequency range of 2 MHz to 250 MHz. Manganese-zinc ferrites are generally used in inductor applications where the operating frequency is less than 5 MHz. The exception is common mode inductors where the impedance of MnZn material makes it the best choice up to 10 MHz
What’s Different between Mixes?
The “Mix” is the chemical formula of the iron oxide. Ferrite is a ceramic consisting of iron oxide and generally either of two types:
Manganese-zinc (MnZn) available as Mix #31, #75 and #77 (and others) – work well for common mode chokes
Nickel-Zinc (NiZn) available as Mix #43, #52, #61, (and others) – generally preferred material for baluns/ununs
Palomar Engineers uses mix 31, 43, 52, 61, 73, 75 and 77 for most applications from RFI/EMI common mode suppression, multi-ratio toroid baluns and ununs and sleeve baluns for line isolation. Each mix number has a measurable permeability and suggested frequency range for certain applications.