When are tags included in the HTML title of the question page?

Question

I see that there are two ways to derive the HTML title from the question. Here some "random" examples.

Tag-prefixed question:

• c - format string vulnerability - printf - Stack Overflow
• classloader - java Class Loader - Stack Overflow
• java - Eclipse WTP vs sydeo, " serves modules without publishing " - Stack Overflow

Solely the question:

• printf vs cout in C++ - Stack Overflow
• Measuring Documentation Coverage with Doxygen - Stack Overflow
• How to build Visual Studio 2012 Publishing Profiles on a Build Server with MsBuild without installing Visual Studio 2012 - Stack Overflow

It's of course relevant for googling questions, so what are the rules behind it? I read something about "the most relevant tag" - but since this may vary depending on the question text, can it be decided automatically?

I'm neither for nor against tags in titles, I only want to understand the logic behind it.

Answer 1

The most popular tag in the question is added to the HTML title, if it isn't found in the actual question title.

c - format string vulnerability - printf - Stack Overflow

C isn't found in the actual title, add it as a prefix.

Measuring Documentation Coverage with Doxygen - Stack Overflow

Doxygen is mentioned in the title, no need to add it.

classloader - java Class Loader - Stack Overflow

Java is mentioned in the question title itself, so check for the next most popular tag in the question: classloader isn't mentioned (note the white space) so add it to the title.