Replacement Characters: U+FFFC–U+FFFD

U+FFFC. The U+FFFC object replacement character is used as an insertion point for objects located within a stream of text.
All other information about the object is kept outside the character data stream.
Internally it is a dummy character that acts as an anchor point for the object’s formatting information.
In addition to assuring correct placement of an object in a data stream, the object replacement character allows the use of general stream-based algorithms for any textual aspects of embedded objects.

U+FFFD. The U+FFFD replacement character is the general substitute character in the Unicode Standard.
It can be substituted for any “unknown” character in another encoding that cannot be mapped in terms of known Unicode characters.
It can also be used as one means of indicating a conversion error, when encountering an ill-formed sequence in a conversion between Unicode encoding forms.
See Section 3.9, Unicode Encoding Forms for detailed recommendations on the use of U+FFFD as replacement for ill-formed sequences. See also Section 5.3, Unknown and Missing Characters for related topics.