Two different relationships, often confused: breaking work down, and recording what has to happen first.

Subtasks — breaking work down

A record can have a parent, which makes it a subtask. Use it when one piece of work is genuinely made of smaller pieces: a launch made of a landing page, a campaign and a press note.

The parent stays the thing people talk about; the children are how it gets done. A rollup on the parent can count how many children are finished, so progress is visible without anyone reporting it.

Blocked by and Blocking — what waits on what

A record can be blocked by another, and therefore blocking something else. This is sequence, not composition: the photo shoot is not part of the packaging design, but it cannot start until the design is approved.

Why bother recording it: when a date slips, the dependency shows you what else moves. Without it, that discovery happens the week the deadline is missed.

How do I turn on Blocking and Blocked By?

A sheet admin opens Config → General and switches on Dependencies. Worksheets and Tasksheets have it. This adds the Blocking and Blocked By fields to the sheet.

Creating a Gantt view turns dependencies on by itself.
While the sheet has a Gantt view, they can't be turned off. The switch says "A Gantt view uses these fields. Remove it to turn dependencies off."
Turning dependencies off hides both fields but keeps the links. They come back when you turn dependencies on again.

To set one, click the item's Blocked By (or Blocking) cell and pick items from the same sheet. Each linked item shows as a chip; click its × to remove it.

The Dependencies setting under Config, General in Atwork: switched on, with the Blocking and Blocked By fields and the note that a Gantt view uses these fields

Which one to use

"This is part of that" → parent / subtask.
"This can't start until that finishes" → blocked by.

They are independent — a subtask can also be blocked by something in another project entirely.

Seeing it

A Gantt view is where dependencies pay off: bars in sequence, and an obvious ripple when one moves. See Gantt and timeline views.

Notes

Do not break work down further than you will actually track. Five real subtasks beat forty nobody updates.
A blocked item is worth surfacing: filter a view to blocked and it becomes your unblocking list.

Subtasks and dependencies