We did not reinvent the most basic standards for Website Baker and arbitrarily define some rules, but we use basically the same set of rules as many other notable projects and large frameworks (see PHP-FIG Referenzliste).
The basic standards for Website Baker are the standards PSR-0 / PSR-1 / PSR-2 and PSR-4 of the PHP Framework Interop Group.
Some might feel uncomfortable with the quite strict wording using the expressions MUST! or MUST NOT! etc. However, these terms are taken exactly as they are from the original PSRs which in turn striclty stick to RFC 2119 and should therefore be understood exactly in this sense.
The most important keywords defined in RFC2119 are:

• Autoloading Standard (PSR-0) - This standard aims to provide a standardized file format, as well as class name and namespace conventions that enable plug and play code. As of 2014-10-21 PSR-0 has been marked as deprecated. PSR-4 is now recommended as an alternative.
• Basic Coding Standard (PSR-1) - This standard helps to achieve the highest possible degree of compatibility of PHP code contributed from different sources.
• Coding Style Guide (PSR-2) This provides instructions that ensure that PHP code always has a standardized visual appearance.
• Improved Autoloading (PSR-4) -A more modern interpretation autoloading, which reflects the continued progress in the ecosystem.
• WebsiteBaker-Adaption - General adaptation of PSR standards to specific conditions of Website Baker.

With every subsequent release of WebsiteBaker, the programming style evolves continually away from the previous 'anarchic' style, widely used for almost 10 years, towards to a more modular, but also forcibly bound to strict programming interfaces manner. This requires a consistent rethinking especially for addon programmers.
At present, i.e. before 2.10.0, compliance with many regulations is still voluntary, in the 2.12.0 much is already deprecated, and later on many of the specifications of 2.12.0 will be mandatory. All this sounds very restrictive to many, but this is gladly accepted, since precisely these limitations will eventually ensure a stable modularity, flexibility and interchangeability for stable, reusability and mainly maintainability of code.

.. which should should, regardless of WB, apply as well to any at least semi-professional project and be a basis of every serious programmer training.

Addons …

1. … May trigger neither the core nor other addons, but are generally triggered by the controlling core
2. … Must not interfere with the flow of the entire system
3. … Are normally not allowed to establish any direct connection to the outside world
4. … May never modify or write data to areas of other addons or the Core
5. … Never change the code of other addons or the core (also applies to installation and upgrade)
6. … May only send data to the browser throught the responder of core
7. … May not define global variables or global constants.
8. … May never use indirect addressing or allow that!

for later versions (2.12), further restrictions will come. See the respective sections of this documentation.