Please someone tell me if I am right or wrong about what "earmarks" are?

To me and earmark is either add ons which would be like Congress providing funds for something and an add on would be for example: $75 million is appropriated for something and an add on would be for example: Congress adding an additional $10 million to it and make it in specific locations. Now there are other... show more To me and earmark is either add ons which would be like Congress providing funds for something and an add on would be for example: $75 million is appropriated for something and an add on would be for example: Congress adding an additional $10 million to it and make it in specific locations. Now there are other earmarks that can be the same amt. of money that is not changed, but Congress can put various restrictions on the bill. Do I have it even half way correct? If I'm wrong please help me understand.
Update: Jay S-Well, thank you. I did know about the earmarks in cattle as I was raised on my Grandfather's horse & cattle farm and owned the stockyards in the county. But actually since this question is in Politics and Govt., I was talking about the political definition.
Update 2: Jack P-Well, then would a rider be like a "carve out" earmark? It sort of sounds like the same thing. I know there are add on earmarks and carve out earmarks. Just trying to figure this out.
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