Metadata IPTC Core Instruction

    var sels = app.document.selections;
    for (var i = 0; i < sels.length; i++) {
        var md = sels.synchronousMetadata;
        md.namespace = "http://ns.adobe.com/photoshop/1.0/";
        md.Keywords = ...
        md.Caption = ...
        md.Title = ...
................

Metadata IPTC Core

Keywords <==> md.Keywords

Description <==> md.Caption

Title <==> md.Title

Instruction <==> ???????????