It is quite possible for the asset types (of tools being integrated) to have endemic fields native only to themselves. In other words, a tool may have a field for which you may not have an equivalent field to map on the other tool.
For example, a Portfolio Item in Agility (mapped to an Epic in Jira) has a "Category" field that further classifies the Portfolio Item into various types such as Epic, Feature, Capability, and so on. However, Jira has no such sub-classification of Epics, which means the "Category" field in Agility can never be mapped with another field in Jira. The "No Field" mapping comes in handy in such cases.
In this case, you can map the Agility's "Category" field to "No Field" (a read-only field) and select a default value for "Category". Once done, any sync event that originates from Jira to Agility takes the predefined default value for the "Category" field and updates the Agility's asset accordingly.