[Oracle 10G] what is "SYS_IOT_TOP_102806", when I cancelled to rebuild the index

We have inserted and deleted many data every days (actually insert all time).

We wanna high performance to insert data. So We have used 32K block.

But We fould tablespaces,that had very very large with a little data(becuase we have deleted every day).
Oh How can?

I had full 100% on that tablespace (but store only index).
I throught to rebuild index, and rebuilded. Oh no I'm foolish, SO I cancelled. (Index need 20% for rebuilding, I forgot .....)

This Problem I should
1. shink compact on index (no affect)
2. shink again (no compact)
-----
Datase will analyze new size on index.
----
3. rebuild index online

But I could't resize datafile, because I fould SYS_IOT_TOP_xxxxx (IOT index) on this tablespace.

I did everything, But It still appear.
But I rebuild index (no online), It(SYS_IOT_TOP_xxxxx) disapear.[Wow!]

So I could resize datafile.

Wow, everything OK again.