Starting from version 7.0, TFS4JIRA supports links synchronization between JIRA and TFS/VSTS. It allows users to map link relation such as parent/child, relates to/related to, custom links, etc. The following would be the steps for links mapping,
Links mapping can be enabled and set in field mapping screen in the sync profile.
Select one Jira Link Type and the corresponding TFS Link Type, and then click Map.
TFS/VSTS does not consider the action of creating links or updating links or removing links as an update of the workitem. Therefore, TFS4JIRA Synchronizer will not be able to detect the changes. The links would be synced when there are any updates at the workitem such as field's value change.
Links which point to issues/workitems outside the project in sync profile will not be created/updated, and there will be warning messages showing in Logs.
Direction of links
There are directional and non-directional link types in both Jira and TFS/VSTS. You may switch the direction of links by clicking the switch button 
at the right of mapped TFS Links type.
There are directional and non-directional link types in both Jira and TFS/VSTS. If there is a mapping between a directional link type and a non-directional link type, TFS4JIRA Synchronizer would not be able to identify the direction, and the direction will be synced randomly.