How did the system understand which path to take or how to differentiate documents? What did it rely on?

The system distinguished documents based on certain key fields or metadata, which served as identifiers or types. For example, when processing different types of documents, fields like "documentType" or a unique identifier indicating the specific processing path could be used.

In Java, this is often implemented through conditional statements or design patterns, such as a factory that selects the appropriate class for processing based on the field value. Annotations or configuration files where routing rules are specified can also be used.

Example:

switch(document.getType()) {
  case "invoice":
    processInvoice(document);
    break;
  case "order":
    processOrder(document);
    break;
  default:
    processDefault(document);
}

Thus, the system "hooked" onto certain fields or features of the document to determine the further processing path.