baler has two different strategies for optimizing JavaScript delivery
The "core" bundle contains the code that baler can determine is critical to the first render of the page. This includes the Magento core libraries, along with some theme-specific and feature-specific code.
The JavaScript included in the "core" bundle only includes dependencies that can be statically-analyzed from a theme's requirejs-config.js. However, many parts of the Magento front-end are controlled by widgets that are specified using a declarative notation in .phtml files.
This is where Graph Preloading comes in. When baler is run, it crawls the file system and determines which .phtml are eligible to be used in a specific area (frontend/adminhtml/base). These templates are then analyzed for any AMD module dependencies.
When a graph of dependencies has been collected for each .phtml file, the lists are flattened and deduped against the "core" bundle. Then, when a shopper requests a page of your store, preload tags are injected that instruct the browser to immediately begin fetching necessary dependencies.