what means "I, O, U" in column ACTIONS from "yum history" ???

hello, I get this:Code:[root@pepsi ~]# yum history
Loaded plugins: fastestmirror
ID | Login user | Date and time | Action(s) | Altered
-------------------------------------------------------------------------------
20 | Cloud User <centos> | 2020-08-31 13:50 | Update | 2
19 | Cloud User <centos> | 2020-08-31 13:50 | I, U | 5
18 | Cloud User <centos> | 2020-08-31 13:50 | Update | 1
17 | Cloud User <centos> | 2020-08-31 13:50 | I, U | 11 EE
16 | Cloud User <centos> | 2020-08-31 13:49 | Install | 1
15 | Cloud User <centos> | 2020-08-31 13:49 | Install | 1
14 | Cloud User <centos> | 2020-08-31 13:49 | Install | 1
13 | Cloud User <centos> | 2018-06-15 15:22 | Erase | 1
12 | Cloud User <centos> | 2018-06-15 15:20 | I, O, U | 281 EE
11 | Cloud User <centos> | 2018-06-15 15:19 | I, U | 22
10 | Cloud User <centos> | 2016-11-10 08:41 | I, U | 3
9 | root <root> | 2016-01-21 06:39 | Install | 31
8 | root <root> | 2016-01-21 06:37 | Install | 1
7 | root <root> | 2016-01-21 06:19 | Install | 1
6 | root <root> | 2016-01-21 06:09 | Install | 12
5 | root <root> | 2016-01-21 06:09 | Install | 1
4 | root <root> | 2016-01-21 05:58 | Install | 34
3 | root <root> | 2016-01-21 05:56 | Update | 25
2 | root <root> | 2016-01-21 05:54 | Install | 1
1 | System <unset> | 2016-01-21 05:45 | Install | 279
history list
[root@pepsi ~]#what means "I, O, U" ???

thanks