Tongyong Zi (universal characters) refers to interchangeable characters, and it includes interchangeable characters with the same pronunciation, interchangeable characters with the same meaning and interchange of ancient and modern characters.
An interchangeable character with the same pronunciation refers to a homophone or character with similar pronunciation that can be used to replace another character and this replacement is in common use.
An interchangeable character with the same meaning refers to interchange of characters that mean the same. For example,
and 
are not interchangeable in most cases, but they will be interchangeable when they both mean talent. Therefore,
(good talent) can also be written as 
.
Interchange of ancient and modern characters means that some Guzi (ancient characters) and Jinzi (modern characters) can be interchangeable. For instance, 
is a Guzi, and 
is a Jinzi. They both mean catch and are interchangeable in certain cases. 
is a Guzi, and 
is a Jinzi. Many ancient intellectuals used but modern people use to mean value.
In a word, interchangeability of words or characters is only applicable in certain contexts and under certain conditions.
（Source: chinaculture.org）
