Dear All,
I am new to Linux World.
For one of the Application which we are going to deploy in Linux, it says that the Application supports Red Hat EL 6 (UL1+) version. Here would like to know what is meant by UL1+.

RedHat Enteprise Level (commonly abbreviated RHEL) has major releases such (currently supported):
RHEL5
RHEL6
RHEL7

Each of the major releases has update subversions so:
RHEL5.0 = First RHEL5
RHEL5.1 = First update of RHEL5
RHEL5.9 = More recent update of RHEL5 (with other 5.x versions having existed between 5.1 and 5.9 and others after 5.9 such as 5.10).
RHEL6.0 = First RHEL6
RHEL6.1 = First update of RHEL6
RHEL6.5 = More recent update of RHEL6 (with other 6.x versions having existed or yet to exist).

Typically the RHELx.0 version being the first of the major release is the buggiest one so the RHELx.1 usually is more stable.

I believe the vender is telling you with UL+1 to have RHEL6.1 or greater.

Typically if you install RHEL6 then run "yum update" with no flags it will update you to the latest version of RHEL6.x. (Which is to say you could install RHEL6.2 for example, run "yum update" and get to the latest RHEL6.x version.) Note that RHEL updates require you to have subscriptions at RedHat's site and have registered the system you wish to update with RedHat.

You can see your release by typing "cat /etc/redhat-release".

Note that RHEL7.1 is already available and is using 3.x kernel and systemd. If the vendor is specifying RHEL6 it may or may not work on RHEL7 - you'd have to ask the vendor. If possible you'd want to run RHEL7 just because it will have a longer lifespan and be more mainstream than RHEL6. However, RHEL6 isn't going away any time soon so if the vendor requires RHEL6 you should feel comfortable using it.
You should avoid RHEL5 because even though it is supported still it is nearing the end of its support life.