What is an APL file?
A file with .apl extension is a Map layer file used by ESRI ArcPad application software. It contains information about a subset of map layer properties for an exported ArcGIS map. It is used by field-based personnel to capture, analyze and display geographic information in the form of raster and vector images. It includes symbology for displaying features, scales, and label expressions for the map layer.APL files can be opened with ArcPad Pro on hand held devices used in the field work.
APL File Format
APL are binary GIS files and its file format information is not available publicly.