The phrase "a system used to" is correct and usable in written English.
You can use it when you are introducing a system that is used for some specific purpose. For example: "A system used to track customer orders was introduced to the company last year."
"You have a system used to target the government's nonviolent political enemies".
LTV is a system used to prevent equipment from being powered on when someone is working on it.
Code from CKAN can then be synced with Drupal, a system used to publish web content, and serves as Data.gov.uk's "front end".
In 2016, for instance, US journalists argued that a system used to assess the risk of future criminal activity discriminates against black defendants.
For example, Namecoin is a system used to create and exchange domain names: the coins contain information about the domain names themselves.
This review describes a system used to test antisense oligonucleotides to over 2000 genes in multiple cell-based assays.
Ubisense is a monitoring system used to evaluate position using a remote triangulation device using tags and sensors.
Loading rack means a single system used to fill tank trucks and railcars at a single geographic site.
But in testimony before Congress last spring, a senior official from Mr. Negroponte's office, Daniel Butler, described a "triage" system used to sort out material that should remain classified.
One shortcut is to use pinyin, a phonetic system used to transliterate Chinese words into the Latin alphabet.
According to the document, Labour has analysed 80% of its party's membership using Mosaic, a classification system used to categorise people into different social bands.
