Clearly the 115 narrative was a PR stunt, but references to 'charges' or 'prosecution' or whatever are simply a convenient shorthand for referring to what's happening here. That involves a body instituting disciplinary proceedings against a member as a result of the latter's alleged large-scale breaches of the body's rules. It's asking a tribunal to confirm the fact of the breaches and impose a suitable punishment where the breaches are upheld.
The PL leadership are interested in causing as much damage to City as they possibly can, including ideally (from their point of view) by achieving an outcome that finishes the club once and for all as a major player in the English game. Egged on by the most powerful member clubs, who hold a disproportionate sway in these matters, they've conducted a fishing expedition of an investigation and seem to have approached the 'charging' aspect in the spirit of throwing as much shit as possible, which will not only damage the club's reputation but might also - you never know - see some of it stick and justify strict sanctions.
Based on what's in the public domain, we may not think they have an especially strong prospect of success. However, people with much greater knowledge than I say there's never a completely sure thing in litigation. And the pedigree (and cost) of the legal representation they've engaged and the length of the hearing both suggest that they're giving it as good a go as they can. I think the PL's conduct is invidious, but there you go.