TL;DR: there’s a new .field container, and .control has been re-purposed.
Since the beginning, the .control has been a very versatile element that allowed:
to space controls vertically
to include an icon alongside inputs, buttons, textareas…
to append a loading spinner
to create horizontal forms
to create control addons
to create control groups
The .control element acted as both a block container (for spacing, for other controls in a horizontal form), but also an inline container (for adding an icon, a loader, an addon, and grouping).
It led to issues where you couldn’t:
add a help text horizontally
add multiple icons or loaders in a group of controls
add a different icon on addons
The new .field element becomes the block container for .control elements. As a result, it inherits the .has-addons, .is-grouped, and .is-horizontal modifiers.
Furthermore .control element can only contain a .button, .input, .select, or .textarea, and eventually a .icon. It can no longer contain a .help element or other .control.