Let's say we start with the following character sequence and cluster values:

A,B,C,D,E 0,1,2,3,4

We then map the characters to glyphs. For simplicity, let's assume that each character maps to the corresponding, identical-looking glyph:

A,B,C,D,E 0,1,2,3,4

Now if, for example, `B`

and `C`

ligate, then the clusters to which they belong "merge".
This merged cluster takes for its cluster number the minimum of all
the cluster numbers of the clusters that went in. In this case, we
get:

A,BC,D,E 0,1 ,3,4

Now let's assume that the `BC`

glyph decomposes
into three components, and `D`

also decomposes into
two. The components each inherit the cluster value of their parent:

A,BC0,BC1,BC2,D0,D1,E 0,1 ,1 ,1 ,3 ,3 ,4

Now if `BC2`

and `D0`

ligate, then
their clusters (numbers 1 and 3) merge into
`min(1,3) = 1`

:

A,BC0,BC1,BC2D0,D1,E 0,1 ,1 ,1 ,1 ,4

At this point, cluster 1 means: the character sequence
`BCD`

is represented by glyphs
`BC0,BC1,BC2D0,D1`

and cannot be broken down any
further.