The composing feature of Flutter's text editing doesn't seem to be explained anywhere in the docs. The docs linked above simply state:

The range of text that is still being composed.

What this feature does is to support the IME feature that allows replacing the text currently being edited with some replacement. This is used, for example, by non-phonetic writing systems:

And even by Gboard to do suggestions (note the underline in the field and suggestions in the keyboard):

We should explain this in the docs!