The in-game editor is a lighter version than the main editor.
You can edit a players or staff members attributes, you can move a player from your club to another and vice versa, (eg. go on Messi's profile and select the editor tool. You'll see "Move to your club" or "Move to another club") You can change a whole teams morale, make their tactical fluidity full, remove injuries and bans. You can also change a players whole identity, from their nationality, to their number of international caps that they have had and position they play. Name, age, the language they speak, their favoured personnel and favourite/disliked teams. Their wages and how long their loan deal is. A personal favourite of mine is the "Become unsackable" option for yourself, as well as the ability to change your own profile, too. Attributes, age, favoured and disliked teams and personnel, nationality and more.
In comparison to the main editor, you can't swap teams in different leagues, create, bring back from and retire players. I think that's about it, really.
So basically The in-game editor you can transfer players etc where as in the main editor you can create teams, leagues and players? Also what about team transfer budgets I want to see what happens if a billionaire took over a Vanarama South Side so would I need the in game editor to edit the budget and chairman or can you do that in the editor? And aswell as that what about swapping teams around such as Reading (my favourite club) into the Premier League will I need to buy the in game editor or will that come with the main editor/database editor thanks