Cisco does a fantastic job obscuring this information for some reason. To answer your specific questions:
- System Version
188.8.131.52900-7 is actually CUCM Version
7.1(5b)SU4. You'll need to download System Version
184.108.40.206901-1 or higher, to get the OS and Application fixes that are found in CUCM Version
- I wouldn't go so far as to say that there is always a 1-to-1 relationship between the two formats (CUCM Version vs System Version). However, in my experience, when Cisco releases a new System Version, they also increment the CUCM version number.
- In older versions of CUCM, there was a much greater disconnect between "OS Fixes" and "CUCM Fixes", which would give you the disparity that Michael Luo describes in the page you linked. However in recent times (post version 7.1 days), all of the recommendations that I have received from Cisco TAC, seem to indicate that the higher the number, the better, period.
The only exceptions that I have encountered, are unreleased "Engineering Special" versions; special releases that are usually only distributed by TAC to fix a very specific and uncommon bug. In the event that you are on an "Unreleased ES" version, you should contact TAC for upgrade support anyway, as there may be undocumented bugs related to upgrading from that version to a normal, mainline release.
The longer answer is that we can piece together a good picture using information from the two sources you linked. They are actually my two "go to" locations for deciphering this information.
First, according to the page you linked from Michael Luo over at UC Corner, we get a picture of how Cisco is structuring the CUCM/Unity Connection/Presence/UCCX build numbers:
On each sub-version, there are also "build-numbers". e.g. 220.127.116.110, 18.104.22.1680, etc. Build-number is the most confusing part.
Generally speaking, build numbers should increase in 1000, such as 22.214.171.1240, 126.96.36.1990, etc.
CUCM is built on Linux OS. Whenever Cisco release an OS security patch, they'll increase the build number by 1000. This is called PSIRT patch.
Remember CUCM is an application running on Linux. OS patch does not contain any CUCM bug fixes. Any bug fixes would be in ES (Engineering Special). ES versions would be indentified by the last three digits in build numbers (e.g. 188.8.131.522)
Secondly, from the CUCM Software Compatability Matrix PDF, (sourced from Cisco Support documentation here), we can find a rough guide of what CUCM Version Number matches which System Version number. This has always in my experience proved to be an accurate guide.
CUCM Version | System Version
9.1(1a) | 184.108.40.20600-5
9.1(1) | 220.127.116.1100-11
9.0(1) | 18.104.22.16800-37
8.6(2a)SU3 | 22.214.171.12400-10
8.6(2a)SU2 | 126.96.36.19900-9