What "enclosed" means probably anybody knows. If not, we'll explain it immediately below. There's nothing difficult about it. What is, anyway, questionable and often confusing, is the correct spelling of this word. If you check literature and publications from different periods of time, you will find both spellings: "enclosed" and "inclosed". But if you check today's dictionaries, you might actually not even find "inclosed" mentioned. So is the latter a misspelling, is there any rule you don't know about, or how do you know when you should use "enclosed" and when "inclosed" is better?
The explanation is easy, don't worry, you'll figure it out right away. Taking a look at the explanation below is enough.
Enclosed vs. Inclosed
"Enclosed" is the word used today most frequently when referring to sending something else than the main document/paper, in the same envelope. This is the main and basic meaning of the verb "to enclose" and, implicitly, of the word "enclosed". Even though we don't use "enclose" that often anymore, because, obviously, traditional letter writing has been replaced with emails and virtual messages, it is still important that you know you have to use "enclosed" if you are in the situation of sending another thing in the same envelope.
As for "inclosed", this is never used anymore. It was once accepted, about two centuries ago, but not anymore. "Inclosed" is old, out of use, and this is also why you won't find it in any notorious dictionary today. Officially, nowadays "inclose" and "inclosed" are considered misspellings of the original word, "enclosed".
When do we use "enclosed"?
As already mentioned, not very often, given the current context of electronic communications. But when we do use it, it defines the action of "sending something in the same envelope as something else" - this is the official explanation. Occasionally, "enclosed" can also be used as an adjective, describing an isolated, closed place, separated from the rest.
Example 1: Be careful with that envelope, please, I have enclosed my letter with all my diplomas in it! - "enclosed" refers to attaching something to another document in the same envelope.
Example 2: I hate enclosed spaces, I feel I'm suffocating if I spend too much time in such places. - as an adjective, "enclosed" can describe isolated places, separated from others.
When do we use "inclosed"?
These days? Never. "Inclosed" is out of use, misspelled, not present in dictionaries and never recommended, especially not in today's vocabulary.
As you can see, there's really not much to remember about these similarly spelled words. No complex definitions, no exceptions from main rules, nothing to confuse you. All you have to keep in mind is that "inclosed" is an old version, today considered a misspelling of the word "enclosed", and you should only use the latter in your expressions.