The term full-expression is defined in [basic.exec.intro.execution] p5

A full-expression is

• an unevaluated operand,
• a constant-expression ([expr.const]),
• an immediate invocation ([expr.const]),
• an init-declarator ([dcl.decl]) or a mem-initializer ([class.base.init]), including the constituent expressions of the initializer,
• an invocation of a destructor generated at the end of the lifetime of an object other than a temporary object ([class.temporary]) whose lifetime has not been extended, or
• an expression that is not a subexpression of another expression and that is not otherwise part of a full-expression.

None of the above bullets is related to "initialization". In my opinion, It seems the term "initialization" is a sequence of expressions whose invocations would initialize an entity. Although, "initialization" has no formal definition in the standard. Consider this example

struct A{
    constexpr A(int){}
};
struct B{
   constexpr B(A const& v):v_(v){}
   A v_;
};
constexpr B b = A(0);  //#1

According to the definition for full-expression, the full-expression in the declaration at #1 should be its init-declarator(namely, b = A(0)).

The [basic.exec.intro.execution] p5 also says

For an initializer, performing the initialization of the entity (including evaluating default member initializers of an aggregate) is also considered part of the full-expression.

Hence, "the full-expression of its initialization" at #1 refers to the init-declarator? [expr.const] p2 says

the full-expression of its initialization is a constant expression when interpreted as a constant-expression

While the definition for constant expression is defined at [expr.const] p11

A constant expression is either a glvalue core constant expression that refers to an entity that is a permitted result of a constant expression (as defined below), or a prvalue core constant expression whose value satisfies the following constraints

Anyhow, the constant expression should first be an expression on grammar. A full-expression is a concept that is convenient to name a language structure that is not necessary to be an expression. As aforementioned, the full-expression is an init-declarator, how could it be a constant expression?

The full-expression at #1 comprises the implicitly call of B::B(A const& v), temporary materialization conversion on A(0) and the destruction of that temporary object. I think the intent is saying that these expressions should satisfy the requirement of a constant expression. However, the current wording is obscure to read and a bit contradictory. The same issue https://stackoverflow.com/questions/62253964/what-is-full-expression-of-initialization