Short answer: Journal selection helps identify potentially suitable journals before you decide where to submit. Journal submission support helps prepare the manuscript files and required documents for a journal you have already selected.
Selection work may assess subject fit, article type, audience, scope, indexing preferences, publication model, and author requirements. Submission work may cover formatting, cover letters, declarations, file naming, figures, and checklist preparation.
Some authors need both services, but they should still be described separately so you know what you are paying for.
Neither service can control editorial decisions or guarantee publication.