Express Server can reproject your source imagery to other EPSG-defined coordinate reference systems (CRS, sometimes called spatial reference systems or SRS). This can be achieved through elements added to the catalog configuration file (catalogs.xml). These elements can either be set per catalog/layer or globally.
For example, let's say you have source imagery in UTM Zone 10N and want to support requests for that layer in both its native CRS and also 4326 (Lat/Long). You can add the following line to your catalogs.xml file:
<wms:CRS>EPSG:4326</wms:CRS>
If you add this element inside of a catalog, then only that catalog supports that output CRS for that layer. If you add the element in the global section of the catalogs.xml file, then all defined layers will support 4326 as an output CRS.
You can also add more than one CRS by using multiple instances of the element as follows:
<wms:CRS>EPSG:4326</wms:CRS>
<wms:CRS>EPSG:3857</wms:CRS>