V$LOCK_TYPE is a cool view for enqueue

SQL> select * from v$lock_type
  2  where type in ( 'IM','TA')
  3  @pr
==============================
TYPE                          : TA
NAME                          : Instance Undo
ID1_TAG                       : operation
ID2_TAG                       : undo segment # / other
IS_USER                       : NO
IS_RECYCLE                    : NO
DESCRIPTION                   : Serializes operations on undo segments and undo tablespaces
CON_ID                        : 0
==============================
TYPE                          : IM
NAME                          : Kti blr lock
ID1_TAG                       : pool #
ID2_TAG                       : 0
IS_USER                       : NO
IS_RECYCLE                    : NO
DESCRIPTION                   : Serializes block recovery for an IMU txn
CON_ID                        : 0

and the last one is in the docs

https://docs.oracle.com/database/121/REFRN/GUID-758BA52A-B707-4958-AA45-2075D74402F2.htm#REFRN00649

A 'tx' slot is an entry in the undo segment required to commence a transaction.

So without making too many assumptions on *why* you have asked the question, it perhaps suggests you've got a lot of concurrent transactions going on ?