It represents parts of the business that all companies are performing in the same way.
They do not provide any competitive advantage.
They are complex, hard to implement. And it’s a problem that is already solved by someone. So the smart thing to do for business is to utilize it and focus on their core subdomains.
Good example is Stripe. It handles everything that is hard about payment, provides its services as an existing solution for business’s problems.
How to Identify in the Legacy code (Brownfield project)
Look for off-the-shelf solutions, subscription services or integration of open source software that company uses.