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The URL (web address) shown by your browser starts with https because you visit this site using a secured connection. Note that the URI reference (identifier) of TRAY TYPE ASSIGNMENT CLASS (dexpi) to be used in DEXPI-compliant Proteus files starts with http - as shown below:

http://sandbox.dexpi.org/rdl/TrayTypeAssignmentClass

TRAY TYPE ASSIGNMENT CLASS (dexpi)

RDF Properties

"TRAY TYPE ASSIGNMENT CLASS"
"TRAY TYPE ASSIGNMENT CLASS"

Namespaces

rdfs
http://www.w3.org/2000/01/rdf-schema#
pca
http://data.posccaesar.org/rdl/
dexpi
http://sandbox.dexpi.org/rdl/