DT_HOME

DT_HOME refers to the AppMon installation directory. It could be referenced as %DT_HOME% on Windows or $DT_HOME on *NIX or generically <DT_HOME>

It originally refereed to the environment variable that is optional now and can be used for convenience and then should point to the AppMon installation root, for example C:\Program Files\dynaTrace\dynaTrace 7.0 for a default 64-bit AppMon installation on Windows.

Fall-back

The bootstrap part of an Agent tries to save the core into a subdirectory agent/downloads for the following directories in the following order:

DT_STORAGE, if not set: DT_HOME, if not set:DTD_HOME (precursor of DT_HOME), if not set: path of the Agent, tmp, current directory

So, if no environment variable is set you are perfectly fine, because write permissions to agent/downloads were set at install time. If all fails, updates are not available.

Convenience factor

When you set DT_HOME you can use it to construct the path in the Agent connection string. For example, instead of -agentpath:/opt/dynatrace-7.0/agent/lib64/dtagent.dll=name=Java,server=localhost, use -agentpath:$DT_HOME/agent/lib64/dtagent.dll=name=Java,server=localhost.