When you need to categorize tasks or add keywords to them, you can use tags. How does it work in Bitrix24: first you add tags to the task form, then you can use filter above the tasks to find tasks with particular keyword (tag).
How to add tags to Bitrix24 tasks
Open new task form to create a new task (or open existing task for editing) & navigate to More section below the form, the Tag option is listed there among the available options, click on it to unroll:
Once the tags have been added, you can filter your tasks by tags. First, click on Filter to open it & click on Add field & select Tag field in order to add it to the filter: