Designers write by others on drawings and in specs to split work between trades. The phrase tells you the item is not yours. Someone else must still carry it.
Example: a gym equipment spec says electrical power to the backstop winch is by others. The equipment bidder supplies the winch, and the electrician brings power to it.
List each by others item as an exclusion in your proposal. If the documents do not show who carries it, tell the general contractor. That gap can come back to you later.
Takeoff Tool flags items marked by others or owner furnished, and a person decides to exclude them.