There are four circumstances under which HTCondor may evict a job.
They are controlled by different expressions.

Reason number 1 is the user priority:
controlled by the Attribute PREEMPTION_REQUIREMENTS
expression in the configuration file.
If there is a job from a higher priority user sitting idle,
the condor_negotiator daemon may evict
a currently running job submitted from a lower priority user if
PREEMPTION_REQUIREMENTS is True.For more on user priorities,
see the Priorities and UserPrio sections of the manual.

Reason number 2 is the owner (machine) policy:
controlled by the PREEMPT expression in the configuration file.
When a job is running and the PREEMPT expression
evaluates to True,
the condor_startd will evict the job.
The PREEMPT expression should reflect
the requirements under which the machine owner will not permit
a job to continue to run.
For example, a policy to evict a currently running job when a key is hit
or when it is the 9:00am work arrival time,
would be expressed in the PREEMPT expression
and enforced by the condor_startd.

Reason number 3 is the owner (machine) preference:
controlled by the RANK expression in the
configuration file (sometimes called the startd rank or machine rank).
The RANK expression is evaluated as a floating point number.
When one job is running, a second idle job that evaluates to a higher
RANK value
tells the condor_startd to prefer the second job over the first.
Therefore, the condor_startd will evict the first
job so that it can start running the second (preferred) job.

Reason number 4 is if HTCondor is to be shutdown:
on a machine that is currently running a job.
HTCondor evicts the currently running job before proceeding
with the shutdown.