Fixed & variable width national character set

Question

In its discussion of NCHAR and NVARHCAR2 datatypes, Oracle talks about fixed width and variable width database character sets. What are they?
I think the concept of fixed length (i.e. NCHAR) and variable length(i.e. NVARCHAR2) types is a different thing. That I understand.
From Oracle doc:
'If the national character set of the database is fixed width, such as JA16EUCFIXED, then you declare the NCHAR column size as the number of characters desired for the string length. If the national character set is variable width, such as JA16SJIS, you declare the column size in bytes.'
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Answer 1

Fixed width charactersets are those in which each character is defined by a fixed number of bits, say 7 or 8 or 16
Variable width charactersets are those for which either 7 or 8 or 16 bits are used to save space. Let us say a character needs only 7 bits, then only 7 bits will be used in this case, and if another character needs 16 bits, they will be used.
In fixed width all characters have same number of bits, so it has higher space requirement.
These have nothing to do with char and varchar2 data types.....