Somebody will be able to explain this more in depth than me. The slot limit that I have encountered was with Redfish. Basically, you have a minimum like in the mountains and then you have a maximum also. So it being called a slot is basically saying it has to fall between two these two set sizes. It does wonders for the fishery. I just got finished with this post and saw our Ohio friend had already done a better job.
"The gods do not deduct from man's allotted span the hours spent in fishing." Babylonian Proverb