Quick Tatkal app is used to book train tickets (especially in TATKAL quota) on IRCTC website or IRCTC Rail Connect app and it saves the time by filling all the details for you so that you can get a ticket as quickly as possible in TATKAL quota's first come first serve basis.
So in order to automate IRCTC Rail Connect app and autofill the details in the app, accessibility service is needed in Quick Tatkal so that it can access and control the app.
Enabling accessibility service for Quick Tatkal helps it to control other app (in our case, IRCTC Rail Connect app). If you have an IRCTC account and want to book tatkal quota tickets with good chance of getting a confirmed ticket, you can do so in Quick Tatkal.
There are 2 modes of booking : Using IRCTC Website & using IRCTC Rail Connect app. To book using the latter, you need to accept the terms and conditions of accessibility service from the prominent disclosure within the app. For more information, you can check out this video demonstration link for the prominent disclosure
Quick Tatkal does not collect or store any of your personal or sensitive data accessed using Accessibility Service. The service is just used to fill the data automatically in IRCTC Rail Conect app in order to book the train ticket quickly
In Quick Tatkal app, Dashboard → Ticket Booking → New Form, you can fill all the details of booking which would be auto filled while booking the train ticket. In RAILCONNECT tab, when you click on 'Book Now' to start the booking, it will display a prompt to enable accessibility service for Quick Tatkal, with a 'SETTINGS' and 'CANCEL' button. By clicking SETTINGS, it will take to your phone's Accessibility Service settings where you can find Quick Tatkal app in the list of apps using accessibility service and you can enable it.
If you do not wish to enable accessibility service, you would not be able to book tickets by autofilling Rail Connect app. Though you can use WEBSITE option to book the ticket on IRCTC website by automating it.