A directional coupler can be a splitter and a combiner as well, depending on how it is used.
REF Directional Couplers are design as tapped transformers with 1/4 wave reactance inverters to cancel the signal in the reverse direction.
When the coupled port is exposed instead of terminated with some R, then you have a 4 port bidirectional splitter with good isolation and low reflectance when match terminated.
To get wider bandwidths , there are various tricks.
A magnetic transformer is often used with bifilar wound hydbrid ferrite core is used. ( commonly used for TV signals) e.g. using 50 or 75 ohm port impedance.
Above is a 3dB 75 Ohm splitter is also a DC-3 (meaning ideal 3db Directional Coupler , but actually 3.5)
Depending on mutual coupling impedance, the coupler can be a -3dB tap ( - 0.5dB loss) down to a -50dB tap with low instertion loss. Also they can be equal power taps.
This one can split 5kW at one frequency.
Since they cancel forward power if terminated properly, with good isolation ( ~ -30dB ) it can be used to combine two sources or split one source. Isolation is the cancellation of Mutual coupling.