HART multidrop is a daisy chain 'loop/network' of multiple HART devices and the receiver, where each HART device is powered by the current in the loop and where the receiver is a HART multiplexor (MUX) that polls each HART device one at a time. The MUX is usually a Modbus Slave/Server that a Modbus host polls to get the contents of the Modbus registers which are populated with the HART data.
In HART multidrop, the analog loop signal is limited to 4mA, which powers the transmitters. The loop wiring also provides the signal transmission path between the HART MUX and the field devices on the network. The HART values are polled by the MUX, one at a time, each field device in turn at a max rate of a 2-3 reads per seconds. HART addresses are 1 through 15.
HART multidrop reduces AI I/O cost at the controller by not requiring HART enabled AI's, but the trade-off is the cost of a HART multiplexor and its configuration.
The host system (the controller) needs software that can handle the output of the HART multiplexor. Modbus has been common in the past, but the future might see a HART Mux use HART-IP, an Ethernet protocol that can be used by DCS Asset Management software ($25k software). As of February 2019, Moore Industries, who manufactures a HART module that uses HART-IP, could not name for me any software other than $25k DCS asset management software that uses HART-IP. Be certain you understand which host software (for communicating with the MUX) is available on the host and what its requirements are.
HART multidrop throughput data rates are definitely more limiting than Foundation Fieldbus or Profibus. A rule of thumb is two PV's per second in a HART multiloop. If you have 8 devices on the loop, then each the MUX get an update from any given field device once every 4 seconds. That works fine for a tank farm but is too slow on most flow loops.
You don't say why you're considering HART multidrop. If you need multiple outputs from a single transmitter, like a Coriolis meter (mass flow, temp and density), there are break-out boxes that take the HART variables off the 4-20mA signal and provide a 4-20mA output for each variable. Moore Industries model HIM is one such device.
I have read 'opinions' that the HART signal is not as robust as Foundation Fieldbus (FF) or Profibus PA signals in very electrically noisy environments. There are reports of HART being swamped by electrical interference. I have no experience one way or the other.
Be aware that the field wiring is a continuous current loop. If the current loop is opened anywhere, at any point in the loop, the open circuit brings down the whole loop.
Some HART devices can take as much as 45 seconds to initialize when powered up from a cold start.