The following italicized excerpt is from the IBM InfoCenter for IBM Business Monitor 8. This is the direct content as written by IBM:

IBM Infocenter for Enabling Purge & Archive Service  for Monitor 8

Per IBM:

To maintain dashboard performance and ensure that your system runs efficiently, you can purge, archive, export, and import instance data for a monitor model version.
The following scenarios explains the benefits of the purging and archiving function:
Scenario one:

• Your IBM® Business Monitor server has run for a long period of time.
• The dashboard becomes slow due to the amount of data.
• You archive data on a one-time or periodic basis.
• The dashboard’s performance improves because there is less data to report on.
• Optionally, you can move the archived data to an environment dedicated to historical analysis or to a data warehouse.

Scenario two:

• You have run the monitor model for a period of time and have some terminated instances and some active instances.
• You show the dashboard with the existing terminated and active instances, and are satisfied with the results.
• You export the terminated and active instances into the flat files as the backup.
• You want to show the same dashboard later, and so you import the instances.

You can purge instance data and archive it on a one-time basis or set it up as a scheduled service.
Scheduled service can be found under Applications > Monitor Services > Monitor Model > Scheduled Services > Purge and Archive Instance Data. Here you can set up the frequency of running the service.

Click on Version > Model Version. Here you can set up the data you want to purge based on a given number of days, as well as, the location of where to create the CSV files that will contain the purged data.