Fieldset groups related fields under a name. It renders a real <fieldset> and <legend>, so a screen reader announces the section on entering it, and disabled turns off every native control inside at once. The fields are laid out by a Form grid, so a section takes the same density, columns and span.
Put several sections in one Form. Each one fuses its own group of corners, and takes the form's size unless it sets its own.
disabled disables every native control in the section, the way a disabled <fieldset> always has. Use it for a section that does not apply yet, such as a shipping address while "same as billing" is checked.
