What is "Professional" Code?

In your experience (or opinion), what are the differences between so-called professional code and code as a hobby?

I'm thinking of what actual code looks like in this case, rather than the overall result of a software system.

Personally, I'd venture that code that becomes part of shipping or bespoke products can often be highly constrained by compatibility with older products, and maintainability requirements for programmers who may be rotated in and out of roles with some frequency.

Can these restrictions make code better, or do they typically make it worse?