A Custom field can be one of the following depending on its’ use.
The custom field type cannot be changed after it has been saved.
You can enter text freely if you choose "Text", which is similar to the Subject name field of the issue. A part of entered data can be used in the keyword search.
"Sentence" is for multiple sentences, similar to the Comment field of the issue. A part of entered data can be subject to the keyword search.
Only numbers can be entered when "Number" is selected. A range of numbers can be set when searching the issue.
If the data is chosen from a fixed list, "List" should be selected.
"Date" will allow you to choose a date from the calendar. A range of dates can be set when searching for an issue.