In journals submissions, what does "Confirmation of Intent" mean? I am planning to submit a manuscript for a special issue in a journal. The guest editor puts a deadline for Confirmation of Intent. What does this mean? What the editor expects from the authors at this level? are there any samples of such thing?
As @vadim123 said in the comment, this is to estimate the size of the special issue. If it were too small, the journal may decide to:


make a very small special issue;
join special issues from two conferences into one;
cancel the special issue and propose acceptance of the paper through a standard process.


If the special issue is too large, they would typically make it into a supplement.

What should you do? Reply yes or no for your participation in the special issue, and if yes, then the estimated length, like: "We'll provide the full proofs while in the 10-page extended abstract there were obly sketches, it'll be circa 20 pages long." If they wanted more information, they would have asked for it explicitly.