Mutual Explanatories
According to Xu Shen's definition in hisOrigin of Chinese Characters, mutual explanatories are those which share the same semantic radicals and are mutually explainable.
Strictly speaking, mutual explanation is a way to explain the meaning of characters through comparison. There is no new character created in this way. Hence mutual explanation is not a way of composing new characters, but a way of using existing ones.
Phonetic loans
The phonetic loan is another way of using existing characters. Xu Shen defined it as a character which is used in a new sense on account of its pronunciation. In other words, it is an internal borrowing on the basis of pronunciation: a character is used in a new meaning which is expressed by a similar sound in the spoken form. In this way an existing character has acquired a new meaning, but no new character is created. The phonetic loan, therefore, like mutual explanatories is not a way of composing Chinese characters.
At the early stage, there were quite a few phonetic loans in the writing system. As the number of existing characters at that time was limited, many concepts had to be expressed by phonetic loans.
Author: Jessie