When linking bugs to milestones, a Filter is available in the to refine the the search results. After clicking Link Tasks, a pop-up will appear with a list of available tasks to link. Click the Filter button to expand a list of available filters. Most filters will have a secondary drop down to complete the search refinement.
To remove a filter, click the close icon.
Depending on a user's project permissions, they may be able to view all the project tasks or only a subset.