Private - this can be sign posted anywhere for different reasons. If you have an office the door may have a "PRIVATE" sign on it or for a specific room in a venue used for parties, conferences etc. As in, "PRIVATE FUNCTION ROOM".
Staff only - this tells the reader than only employees may enter. You may see it for toilets, stock rooms, etc.
Only authorized personnel permitted - this is quite a serious sign, normally used if there is a risk or is a dangerous area. You can also add more like "Only authorized personnel permitted past this point" which is widely used.
Generally speaking, the same signs widely used around the world with an image detailing what kind of area it is.
Private（プライベート）という言葉は、あらゆる理由で使えます。自分のオフィスがあれば、”Private"とサインを付けることも出来ますし、会議室やイベントなどに使う個室のことを”Private"と表示することも出来ます。例：”Private Function Room"（会議やイベントなど、プライベートな理由で使用する個室という意味）
Staff Only（スタッフのみ）は「関係者のみ」という意味なので、お手洗い（スタッフ専用）や在庫管理室など、社員や従業員のみが入れる場所を表します。
Only Authorized Personnel Permitted（関係者以外立ち入り禁止）は、堅苦しい表現ですが、通常、危険地帯などリスクがあるエリアに使われます。”Only Authorized Personnel Permitted Past This Point”（ここから先は）と付け足すことも出来ます。
世界共通の「関係者以外NG」を表す画像を使用するのも良いかもしれません。
"Staff Only Beyond This Point" is what I would write on a sign, because it tells anyone who is not a member of the staff that they are not allowed to go any further.
"Staff Only, Please" is another, more polite way of saying the same thing.
Note that on signs, the first letter of each word is usually capitalized. This is standard, and it actually makes it easier for people to see and read the sign. It could also be in ALL CAPITALS if you REALLY want the person to notice the sign and not ignore it! :)
私だったら、"Staff Only Beyond This Point"（ここから先はスタッフ以外立ち入り禁止）と看板に書きます。これで、スタッフ以外の人はそこから先に入れないことを伝えられます。
"Staff Only, Please"（スタッフ以外立ち入り禁止）は、同じ意味のより丁寧な言い方です。
看板では普通それぞれの単語の頭文字が大文字で書かれます。これがスタンダードです。こうすることで、看板が目に入りやすく、読みやすくなります。また、どうしても看板に気づいてもらいたい、そして無視されたくないなら、文字を全て大文字にすることもできます！
参考になれば幸いです。
If you are talking about people that work in a business/company then you refer to them as 'staff' or 'employees' so if you are wanitng to say its an area that only staff or employees can go in then you would say
'staff only' or 'employees only' you could also add area if you want to say a certain place
If you want to write that only people involved (such as staff) are allowed to enter a certain place, then you may make up a sign which uses one of the two exampled sentences above.
When you want to write a sign that indicated that a certain entry point is only for staff members, then you may write it in the following ways:
-Staff Only
-Only staff is permitted beyond this point
-Strictly staff entrance
スタッフ以外その入り口を利用できないことを看板で知らせたいなら、次のように書けます。
-Staff Only（スタッフ専用）
-Only staff is permitted beyond this point（これより先はスタッフ以外立ち入り禁止です）
-Strictly staff entrance（スタッフ専用）
The two sentences you see provided above are terrific ways to express that only the staff is allowed in a particular area. In the first sentence you will see the plural form of the noun employee and in the second sentence you will see the plural form of the noun worker. Both of these refer to members of a staff. Both of these words are appropriate for formal settings.
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