Issue tracker plugins are add-ons that a company buys from a marketplace and installs in its issue tracker, to add features that the tracker's maker did not build. They are also called apps or extensions. The marketplace for Atlassian's products alone lists thousands of them, and some companies build and sell nothing else.
Why are they needed? A tracker does the common jobs well: tickets, boards and reports. But every company has needs of its own. One team wants to log the hours spent on each ticket. A testing team wants to store test cases and their results beside the tickets. A finance team wants an approval step before certain tickets are closed. The tracker's maker cannot build all of these. So it opens its system to outside developers through an API, a fixed set of requests that one program can send to another. A plugin uses this API to read tickets, add new fields and show new screens inside the tracker.
Imagine a company in Kochi that makes software for hospitals. Its 200 engineers plan their work in an issue tracker. Its customers, the hospitals, pay for support by the hour. So the company must record the time spent on each hospital's tickets, and send each hospital a monthly report. The tracker cannot do this on its own. The admin buys a time-tracking plugin from the marketplace. It costs about ₹200 per user per month, so about ₹40,000 a month for 200 users. The plugin adds a "Log time" button to every ticket. At the end of the month, it produces a report for each hospital, for example 64 hours logged on one hospital's tickets. The company's accounts team calculates that hospital's bill from the report.
Plugins are billed through the marketplace, together with the tracker itself. A similar idea is a cloud marketplace for resellers, through which telecom operators and distributors resell other companies' software on one bill.
Plugins bring two risks. The first is cost. A company can end up paying for dozens of plugins that few people use. SaaS management software finds every paid add-on, and shows who uses it and what it costs.
The second risk is dependence. After two years, the hospital-software company's billing depends on the plugin's time records. Moving to a different tracker now means moving those records and rebuilding the reports. Many plugins exist for only one tracker. This is one form of vendor lock-in: the more plugins a company relies on, the harder it becomes to change trackers.