On the 2 meter and 70 centimeter band plans published by the ARRL, what is the difference in intended use between the calling frequencies (144.200, 432.10 MHz) and the simplex calling frequencies (146.52, 446.00 MHz)?
The simplex calling frequencies (146.520/446.000 Mhz) are intended for FM simplex communication, while the other pair (144.200/432.100) are for SSB. In general use, the term "simplex" implies FM modulation since FM is commonly used in both simplex and duplex operation. SSB, CW, and other modes are generally used for longer distance, simplex-only communication.
Correction. Simplex is a mode of communication regardless of frequency band or modulation being used. Simplex in Amateur Radio is one way communication at a time between two communicating stations. When simultaneous transmission is used with capable equipment, it's called full duplex, meaning both stations can transmit and receive at the same time. There are other modes of communication as defined by the data communications world outside of Amateur Radio. Simplex by their definition is a device transmitting in one direction only, not able to reverse direction. Half duplex is a single transmission one way at a time, but is able to reverse direction. You could say Amateur Radio Simplex is really half duplex communication.
Simplex implies transmission and reception on the same frequency without using a repeater. The band plan specifies that no repeaters are ever to transmit or receive on calling frequencies, nor should other operating modes, such as cross-band communication or remote control use them. In addition, it is expected that you can make initial contact on the calling frequency, but that you subsequently agree with each other to move the conversation to an alternative frequency (if you are going to speak to each other for more than a few minutes) to keep the calling frequency free for others' use.