The full solution is below, with a short explanation for each group. To avoid spoilers, play the puzzle or open the hints page before reading the answer panels.
HOURGLASS, WATCH, TIMER, and CLOCK all fit the category "Timekeepers". They help measure time. The shared link accounts for all four words, which is why the remaining board words should be tested against a different rule.
Category
Map features
SCALE / COMPASS / BORDER / LEGEND
SCALE, COMPASS, BORDER, and LEGEND all fit the category "Map features". These help you read a map. The shared link accounts for all four words, which is why the remaining board words should be tested against a different rule.
Category
Desk items
LAMP / PENCIL / STAPLER / NOTEBOOK
LAMP, PENCIL, STAPLER, and NOTEBOOK all fit the category "Desk items". They belong within arm's reach at work. The shared link accounts for all four words, which is why the remaining board words should be tested against a different rule.
Category
Photo terms
FOCUS / SHUTTER / LENS / FLASH
FOCUS, SHUTTER, LENS, and FLASH all fit the category "Photo terms". A camera owner knows these. The shared link accounts for all four words, which is why the remaining board words should be tested against a different rule.