Use of the word 'off' to indicate a quantity

Using "off" to indicate quantity is quite common in Engineering jargon. It is quite usual to find such sentences in engineering books, and invoices. Though it is incorrect yet used extensively. It is better to change the sentence as to not use it.

Just one correction in adherence to correct use of numbers in sentences. The correct one is:

The system has one service and two host connections.

Yes this use of off is normal and accepted usage. However, it is almost entirely encountered in technical or contractual documents - particularly purchase orders.

It is not commonly encountered in normal speech.

You will often see items listed in a purchase order:

Cables - 2 off
Connections - 3 off

This usage is a means of emphasizing the quantity to differentiate it from any other number in the item. For example:

Cables type 2 - 2 off
Cables type 3 - 1 off

You do occasionally come across the cumbersome "I need two off of those cables".

While it's entirely possible that the usage is in fact incorrect and your substitute is what is actually meant, I wanted to point out that there is a real and technically correct meaning to the example sentence, where "off" is an preposition linked to the following noun (which is its object), and not related to quantity in any way.

The sentence would be correct English to describe the following situation:

• The "system" manages a computer communication network.
• The system has two connections which reach other computers ("hosts" in network parlance). These are the "off host" connections.
• Of the two "off host" connections, one uses the service being managed ("on service"), and one uses a parallel communication channel ("off service").
• The number of connections inside the computer ("on host" connections) is not specified.

Personally, I would hyphenate off-host and off-service, as although it's not strictly necessary, with hyphenation you would not have been confused.

Before "correcting" the document, confirm the intended meaning.

The system has 2 off host and 1 off service connections is accepted by most readers or hearers because they think it’s some kind of jargon they can basically understand, even if they can’t cope with the full details. You are right; they are wrong but it’s not a mutation of of as in I’ll need 3 of these cables. It is, as you think, simply incorrect and yucky. It does come from request sheets, but only by mistake

3 of and 3 off are different uses from different roots through different routes.

3 of is ordinary English and might refer to packets of cornflakes, bicycles or anything you care to mention. It does matter slightly that three of those doohickies will generally come from stock, not any kind of special order.

3 off is not ordinary English even though many people mistake it for 3 of or think they’re interchangeable. 3 off is correctly used only in fields like manufacturing, printing, baking or bottling and other production situations… never retailing.

I’ve seen every stage of this in building aircraft and their components, making safety helmets and printing periodicals. Different items; different methods; identical language.

Whether in a traditional forge or a small workshop or a new-fangled assembly line, the workspace generally has to be re-organized for each item and a decision taken as to how many examples to… uh… run off. This is true whether I’m turning them out for stock or to meet a wholesale order from a specific customer… which requires such usages as This is a custom run… 200 off.

I might run off one or a dozen proof copies of a newspaper or a light-fitting, then run off as many more as are wanted this time. Thus the in-house trial might need the line to run 3 off and the next day’s production might call for 300 off, meaning nothing more arcane than run 300 off the line.

Anywhere production and retail people or premises overlap both customers and less-specialised staff hear but don’t fully grasp the workshop instructions and mistakenly switch the terms.

Strictly, filling the boxes with cornflakes doesn’t come into this but the box factory might run off 30,000 items per shift while at the other end, the family buys three of those filled boxes.