Help needed to understand JLS §15.14

Can someone please help me to understand JLS §15.14, it says:

Postfix expressions include uses of the postfix ++ and -- operators. Also, as discussed in §15.8, names are not considered to be primary expressions, but are handled separately in the grammar to avoid certain ambiguities. They become interchangeable only here, at the level of precedence of postfix expressions.

PostfixExpression:
Primary
ExpressionName
PostIncrementExpression
PostDecrementExpression

Questions:
1. According to documentation above, "Primary" & "ExpressionName" are also considered as "PostfixExpression". Can someone please explain with some grammar examples?
2. The statement "They become interchangeable only here, at the level of precedence of postfix expressions." What is exactly interchangeable? Where can I find the level of precedence of postfix expressions?

Thank you for your time.