What is a product version? why it changes randomly?

What is a product version? why it changes randomly? For example. Firefox new version is v.3.6.7. their previous version is v.3.6. my doubt is why they don't use v.3.6.1 instead of using v.3.6.7. Is any sp开发者_如何学Cecific reason behind this kind of versioning. or they give it randomly.

Each product chooses the versioning strategy differently, but it's usually something styled in the following manner:

3.1.1.123456

• major version (big API / behavior incompatibilities; huge new features); example - iPhone 3 vs. iPhone 4
• minor version (minor new features, speedups, basic API/behavior is unchanged and usually backward compatible); example - iPhone 3 vs. iPhone 3GS
• release version (bug fixes in a specific minor version)
• Version control tag or build identifier to quickly match the version string to a version control tag.

Any of those do not have to be sequential in number - versions can be skipped for any reason.

my doubt is why they don't use v.3.6.1 instead of using v.3.6.7.

They did release all the previous ones: Mozilla Firefox 3.6