Understanding key fields
A key field provides context for your measurements. For example, measuring Bits Received from your web server can be helpful, but it is extra helpful to categorize those bits received by the application used. Application is a key field that makes this happen.
 
Figure 33: Editing a widget - this is where you select key fields
Apex key field locationApex key field location
 
Most widgets have no context until a key field is added. Most measurements your data sources collect and pass to Apex are just numbers—they are pieces of information. After you give the numbers some context, like a key field will do, you begin to create knowledge. This is the difference between knowing your average VoIP MOS score is low and determining which UC Manager is responsible for lowering the average.
Generally, you can think of key fields as legends, like the legends included in a bar graph. That statement is not always true, though, because it depends on the widget type you choose to use. For example, status widgets do not even use key fields. Here are some examples of where key fields are seen:
Bar graphs by summary—the key fields combine to represent each bar.
Line graphs by time—the key fields combine to represent each line.
Pie charts—the key fields combine to represent each piece of pie.
Tables by summary—each key field is a column, and even the data fields get columns too.
Adding more than one key field is acceptable. Suppose you want to see packet volume over time by application. Your data field would be Packets/sec Total and your key field Application. But what if you want more than just packet volume over time by application? What if it is important to know the MAC address as well? Just add MAC Address - Client and/or MAC Address - Server as key fields, depending on which endpoints are important to you. If you were editing a table widget, the table would begin showing the MAC addresses.
Filters will interact with your chosen key fields, and sometimes that is unwelcome. For example, having a key field of VLAN Tag ID and a dashboard or widget filter that ignores all measurements from VLAN 1, VLAN 2, and VLAN 3, can show you VLAN tag IDs from VLAN 4 or greater only. Maybe this is what you intend, but sometimes it might cause confusion or negatively influence your understanding.