that's a wide subject...
* limit (https://wiki.mikrotik.com/wiki/Manual:I ... all/Filter
) will match as long as conditions as specified are met. And so needs to be followed by rule for "when not".
* it's only one of conditions and needs other to be useful, ex: connection-state=new / tcp.flag=syn + limit => control new connection rate to some resource
* it's a condition and available in all 4 tables (raw,nat,mangle,filter)
you can use it to impose all kinds of rate limiting, not only "new"
UDP is connectionless (https://en.wikipedia.org/wiki/User_Datagram_Protocol
) there is no state (state within RouterOs is based on its own tracking (first/last seen + timeouts), not supported by protocol)
TCP is connection oriented and state is supported by protocol
Limiting UDP works, but he TCP apparently not so much, see viewtopic.php?f=2&t=126354
But that also depends on where it's applied: if in "filter" a lot of logic will already by performed on that packet. If in "raw" with "tcp.flags=syn", nothing else except "hotspot-in"