Substrate Types and Traits

Pallets may access their own associated types as well as the associated types of other pallets in the runtime. This is seen in most pallets when they access frame_system's associated AccountId type. All pallets are tightly coupled to frame_system through this syntax, and thus have access to its types. You may optionally couple to additional pallets in this way.

Another option, loosely coupling to other pallets, is discussed further later.

pub trait Trait: system::Trait {}

This provides access to Hash, AccountId, and BlockNumber anywhere that specifies the generic <T: Trait> using T::<Type>. It also provides access to other useful types, declared in frame_system's configuration trait.

basically add a note here on why traits are important for runtime development => we are in the business of building libraries to support the configuration and modular and extensible digital infrastructure...

support::traits

Unlike in smart contract development, the way to inherit shared behavior is not to directly import other pallets. Instead, it is common to either implement the same logic in the new context or utilize a trait from frame_support to guide the new implementation. By abstracting common behavior from pallets into frame_support, Substrate makes it easy to extract and enforce best practices in the runtime.