[R] Data frame column

[ManInMoon]

I have a big data frame and I have extracted a bit by doing:

> y<-d[1:10,6]
> y
 [1] Headings 0        -49      -98      -49      -41      -120     -155    
-204     -169    
92329 Levels: -0 -1 -10 -100 -1000 -10000 -10001 -10002 -10003 -10004 -10005
-10006 -10007 -10008 -10009 -1001 -10010 -10011 -10012 -10013 -10014 -10015
-10016 -10017 -10018 -10019 -1002 -10020 -10021 -10022 -10023 -10024 ...
Headings
>

What does the "levels" means?

If I create a similar object as below - I don't get the levels message.

> x <-c(3,4,5,6,3,2,1)
> x
[1] 3 4 5 6 3 2 1
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