Result Interpretation of Tukey's Test Box Plot 
I have a question related to the interpretation of the result of Tukey's test box plot. I am attaching the two plots from the R graph gallery, which I am following. According to the Confidence level plot, only AC and DB are not significantly different.
So, the box plot will be both A and C of the same color and letter and DB of the same color and letter. It's a simple case.
My main confusions are considering a scenario where:
Group A I have letter displayed c and for group E I have letter bc and For group C I have b. So what does this letter combination bc means? Does it mean that A and E are not significantly different and E and C not significantly different? I was expecting the shared letter to be on top for example for E on top of b be c, but in my case, it's together and I am really confused. 
Any help to understand how the compact letter display for Tukey's result boxplot functions is appreciated. 

 A: The
documentation of the cld()
function
refers to Piepho (2004), but
even more on point in this context is

Piepho,
Hans-Peter
(2018) Letters in Mean Comparisons: What They Do and Don’t Mean,
 Agronomy Journal, 110(2), 431-434. DOI:
10.2134/agronj2017.10.0580
(ResearchGate)
Abstract

*

*Letter displays allow efficient reporting of pairwise treatment
comparisons.

*It is important to correctly convey the meaning of letters in
captions to tables and graphs displaying treatment means.

*The meaning of a letter display can and should be stated in a
single sentence without ambiguity.

Letter displays are often used to report results of all pairwise
comparisons among treatment means in comparative experiments. In
captions to tables and charts using such letter displays, it is
crucial to explain properly what the letters mean. In this paper I
explain what the letters mean and how this meaning can be succinctly
conveyed in a single sentence without ambiguity. This is contrasted to
counter-examples commonly found in publications.

A:     mean

Group 1  4.7  A
Group 2  5.9  AB
Group 3  7.8   B
Means with the same letter are not significantly different (by some test that you name). Thus the only significant difference is between Group 1 and Group 3. You also provide the significance level, usually 0.05, but other options are sometimes used.
