网上没搜寻到什么systemjs的资料

如下代码，
有几个问题：
1.

"materialize": "npm:angular2-materialize",
"angular2-materialize": "npm:angular2-materialize",           

这两句什么意思?,后面的路径是一样的，前面的名字起什么作用？

2.

 packages: {
            app: {
                main: './main.js',
                // defaultExtension: 'js'
            },

package是干嘛的，注释说是当没有文件名的时候告诉加载器如何加载？这句话怎么理解？

全部代码：

       
/**
 * System configuration for Angular samples
 * Adjust as necessary for your application needs.
 */
(function (global) {
    System.config({
        defaultJSExtensions: true,
        paths: {
            // paths serve as alias
            'npm:': 'node_modules/'
        },
        // map tells the System loader where to look for things
        map: {
            // our app is within the app folder
            app: 'app',
            // angular bundles
            '@angular/core': 'npm:@angular/core/bundles/core.umd.js',
            '@angular/common': 'npm:@angular/common/bundles/common.umd.js',
            '@angular/compiler': 'npm:@angular/compiler/bundles/compiler.umd.js',
            '@angular/platform-browser': 'npm:@angular/platform-browser/bundles/platform-browser.umd.js',
            '@angular/platform-browser-dynamic': 'npm:@angular/platform-browser-dynamic/bundles/platform-browser-dynamic.umd.js',
            '@angular/http': 'npm:@angular/http/bundles/http.umd.js',
            '@angular/router': 'npm:@angular/router/bundles/router.umd.js',
            '@angular/forms': 'npm:@angular/forms/bundles/forms.umd.js',
            // other libraries
            'rxjs': 'npm:rxjs',
            // 'angular-in-memory-web-api': 'npm:angular-in-memory-web-api',
            'angular-in-memory-web-api': 'npm:angular-in-memory-web-api/bundles/in-memory-web-api.umd.js',
            'angular2-cookie': 'npm:angular2-cookie',

            //material
            "materialize-css": "npm:materialize-css",
            "materialize": "npm:angular2-materialize",
            "angular2-materialize": "npm:angular2-materialize",

            // 'ng2-image-lazy-load': 'node_modules/ng2-image-lazy-load'
            'angular2-infinite-scroll': 'npm:angular2-infinite-scroll'
        },
        // packages tells the System loader how to load when no filename and/or no extension
        packages: {
            app: {
                main: './main.js',
                // defaultExtension: 'js'
            },
            rxjs: {
                // defaultExtension: 'js'
            },
            'angular-in-memory-web-api': {
                main: './index.js',
                // defaultExtension: 'js'
            },
            "materialize-css": {
                // "format": "global",
                main: "dist/js/materialize",
                // "defaultExtension": "js"
            },
            "materialize": {
                main: "dist/materialize-directive",
                // "defaultExtension": "js"
            },
            'angular2-cookie': {
                main: './core.js',
                // defaultExtension: 'js'
            },
            'angular2-infinite-scroll': {
                main: './angular2-infinite-scroll.js',
                // defaultExtension: 'js'
            },
        }
    });
})(this);