What do double superscripts mean in an ANOVA table of 'ad hoc' results What is this alphabet (a,b,c) and what does its combination mean in the table?
I have read a lot of literature similar to my work and it shows that most of the researchers have used different superscript letters a, b, c, and d along with the data. It is written "the values with different superscript letters in a column are significantly different (p<0.05)"


In the first row of your table, you have superscripts $c, c, d, b, b, a$ for compositions $A, B, C, D, E, F,$ respectively.
The superscripts mean that $A$ and $B$ are not
significant from each other in a statistical sense.
Also, that $D$ and $E$ are not significantly different. However, significant differences were
found between $A$ and $C,$ between $B$ and $C,$
between $E$ and $F$ (among other differences).
More generally, it is important to understand that "lack of
significant difference" is not the same thing as "exact equality".
In a similar analysis of this kind with $A$ significantly different from $C,$ you may have $B$ with an intermediate value between $A$ and $C$ and
yet not be able to decide from the data whether
to "group" $B$ with $A,$ $B$ with $C,$ or neither.
That's why some of the rows in your table have
items with double (indecisive) superscripts.
