This may be a dumb question, but can someone explain the difference between a "Map Image Layer" and a "Feature Layer" in AGOL terms?
A Map Image Layer is the entire map service. For example, a map service may consist of many layers. When you add a Map Image Layer to a web map, you are not specifying an index value at the end of the URL, so all layers are added.
A Feature Layer is one layer within a map service. For example, you specify an index at the end of the URL.
Map Image Layer
Wasn't there a time when "Map Image Layers" were called dynamic layers??
Yes, in my opinion, those two can be considered the same.
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