minecraft-commandsminecraft-java-edition
I've seen # and !# show up before minecraft:entity when I'm typing a command to kill or teleport all of one mob. What do they mean? I experimented with
/kill @e[type=!#minecraft:skeletons]
and
/kill @e[type=#minecraft:skeletons]
But they both end up with everything dying; not just skeletons, and it includes me. Is this a bug, or am I not using it right? Below is a picture of what I mean:
This is a glitch with Minecraft that it also spawns multiple pigmen from one pig. The easiest way is to disconnect and reconnect. They should disappear.
This has happened in my server.
And it IS a client side bug like SevenSidedDie said.
The possible mutations screen is just that, the possible mutations. It is to let you know how many different combinations are possible with that bee. In case of the first screen shot above the been you have analyzed has 6 possible mutations (change into another bee) of which you have found one.
The Apiarist Chest shows this same information but only the resultant bee instead of what the combination bee and that bee will turn into.
If you have the addon mod 'Extra Bees' then you will have access to a tool that can give you a better run down of the information. It is called the Apiarist Database and is made in a carpenter. It will give you an interface like the following screenshot: 
In the shot above you can see that I have a Noble bee selected and show that one of its found mutations is to a Majestic bee with another found one below that (Imperial) then several unknown mutations. The ? on the bees mean I have never seen that bee type before, the bees that are just outlines means I have found the bee but I have not made the mutation as of yet (Those are the bees in the second row there and when there is only one of them it is usually the selected bee but if you look at the last row you will see I have two bees that should be able to breed to produce a mutation, I just have not gotten it yet.)
Hope this helps.
Best Answer
Minecraft has a set of pre-defined tags (or groups) for entities, blocks, items, etc. These tags allow you to select multiple types of entities (or other mentioned types) at the same time.
The "#" prefix indicates that you are referencing a tag. For example, "#minecraft:arrows" will target all arrow types (normal arrows and spectral arrows). The exclamation mark before the "#" prefix is used in combination with the equal sign to invert the condition.
So "type=!#minecraft:arrows" will target every entity that is not an arrow. You can find more info about tags on this wiki page.