What is a dump channel?
Simply put, a dump (also called an exclusion) channel is used to group and exclude everything that is not of interest for your study.
This typically involves using one or more antibodies to stain antigens that are known not to be expressed by the target cell population. These signals can then simply be gated out of the final analysis, whereas any cells that are negative for the dump channel signal can proceed to further analysis.

What goes in a dump channel?
The benefit of a dump channel is that it can group multiple antibodies using the same (or similar) fluorochrome into a single channel. By combining these exclusion signals into one channel, researchers leave all other channels free for the actual analysis.
For example, consider a cell population with 3 types of cells:

The dump channel should contain antibodies to antigens B and C, labeled with the same fluorochrome, to be excluded upon analysis.
In addition to marking antigens that are not expressed by your population of interest, viability dyes should be included in a dump channel to label dead and dying cells for exclusion. Once again, the viability dye can use the same fluorochrome as the other dump channel antibodies to be incorporated into the dump channel as well.

Why should you use one?
Non-specific labeling is most common for Fc receptor-positive cells, dead and dying cells, and debris. Obviously, this can significantly skew the data and lead to false analysis.
Gating out ambiguous signal is especially important for heterogeneous cell populations to ensure that cell types are being identified properly.

How do I design a panel using a dump channel?
Adding a dump channel to your panel design is easy!
Simply, click on the "Options" button for the antigen you want to place in the dump channel. This will open a slide window showing multiple options available to you for this antigen.

Click the toggle button for "Add to Dump Channel".

You can add as many markers as is necessary to this dump channel. When you continue to the Select Products page you will be able to assign the same fluorochrome to all of the antigens that you have designated for the dump channel.

So, be sure to remember to include a dump channel in your next experiment design! Using these negative controls will help with your gating strategy, and combining them into a dump channel will leave all of the other channels open for you analysis.