First of all – some abbreviation’s explanation:
RTM - Release To Manufacturing
MR - Maintenance Release (replaced by RU)
RU - Release Update
MP - Maintenance Patch
PP - Point Patch
RTM was only the very first version of Symantec Endpoint Protection (SEP). SEPM RTM was 11.0.776.942 and SEP RTM was 11.0.780.1109
MR was the main release of the SEP or SEPM version (started from MR1 and it ended in MR4)
RU in reality means the same as MR but for some reason it replaced MR after MR4. So the next main release after MR4 was RU5 (currently it arrived up to RU6. RU7 is planned in the near future).
MP is a minor release for the specific MR (it never “exists” without MR). There have been already many versions with MP till present. For example: MR2 MP1 (11.0.2010.25), MR2 MP2 (11.0.2020.56), RU6 MP2 (11.0.6200.754)
There are few versions with “a” at the end (for example RU6a) – this is the release with 1 or 2 important updates released just after the last update. The version without “a” is removed then.
Example: just after the MR4 MP1 the MR4 MP1a was released and MR4 MP1 was removed. The same is for RU6 and RU6a.
PP is a small update for MR, RU or MP (examples: MR1 PP1, RU5 PP1, RU6 MP2 PP1)
About the installation:
You can install any fresh version of RTM, MR, RU or MP (both SEPM and SEP) since this is the full and complete release. PP you can install only over a specific MR, RU or MP because PP is only a small patch.
About the migration:
In general you can migrate to every RU or MR version (without MP or PP) from any previous version. Example: you can migrate from RTM to MR1, from MR2 MP1 to MR4, from RU5 PP1 to RU6/RU6a
However it is better to avoid migrations between the versions released within a longer time – for example migrating directly from RTM to RU6/RU6a is not suggested.
If you want to migrate to some MP version, you need first to migrate to its MR or RU version. So migration to RU6 MP2 is supported only from RU6, RU6a and RU6 MP1. Migration to MR4 MP1a is supported only from MR4 and MR4 MP1.
Example: if you are running MR4 MP2 version and want to migrate to RU6 MP2, you need first to migrate to RU6/RU6a and then to RU6 MP2.
Note: You can migrate to the SEP 12.1 from every version except RU7. The migration from RU7 is supported to the version 12.1 RU1.
Migration documents for Symantec Endpoint Protection 11
Support for Migrating from Endpoint Protection 11 Release Update 7 to Endpoint Protection 12.1
What are the Symantec Endpoint Protection (SEP) versions released officially?