Header text is used in legend element of new forms library.
This text is usually read together with the name of the element, in some cases it is skipped (I guess if there are several fields in the group some of them are not read to prevent repetition, I can not find any pattern in it, even-odd rule when tabbing forward first time? - I will have to read the docs one day)
For example, sign-up form first name field is read as
Please supply some information about yourself First name required filed Edit
IMHO the legend should be shorter - it should not give instructions, I would propose something like "User details", the instructions could be in a notice box above the form.
Another example group mode in course settings is read as:
Group mode Group mode Help with Group mode Combo box No groups one of three
the solution should be IMO to use heading "Groups" instead of the repeated "Group mode"
1/ use short descriptive strings for headings
2/ do not use the same string for heading and option name