Transmit flows are 'streams' of media packets which supply data between devices. Dante devices can support a finite number of transmit flows, depending on their specific hardware configuration and firmware version.
Dante supports two types of flows - unicast and multicast.
Unicast flows are used to supply media to specific devices, and support up to 4 channels of audio simultaneously, or 1 channel of video. So if you subscribe 4 audio channels on a receiver to 4 audio channels on a single transmitter, that audio data will be supplied using one flow. If you were to then subscribe a fifth audio channel, a second flow would have to be created.
If you were to subscribe 4 different receivers to the same audio channel on a single transmitter, Dante Controller would have to create 4 unicast flows, because the audio data is going to 4 different places. In this case, it would be better to create a multicast flow.
A multicast flow 'broadcasts' media across the network, so multiple receivers can subscribe to the media, but the transmitter is only using one of its available flows. Multicast flows can be configured with up to 64 channels (depending on the Dante device type). However, multicast media traffic can propagate throughout an entire network, using up bandwidth on every link in the network. 100Mbps links in particular are easily saturated when large numbers of multicast flows exist (and would be overwhelmed by 1 channel of video). Therefore, multicast flows should only be used when there is a good reason to do so.
Note: IGMP snooping must be enabled on network switches that will be carrying Dante video on 1Gbps network infrastructure. Refer to your switch manufacturer's documentation for information about enabling IGMP snooping.
If you then exceed the supported number of transmit flows, any further subscriptions will fail, and the tooltip for the failed subscription will read 'No more flows (TX): transmitter cannot support any more flows, reduce fan out by unsubscribing receivers or switching to multicast.'