HTTP Header Manager obeys JMeter Scoping Rules so:
- If you place HTTP Header Manager at the same level as HTTP Request samplers - it will be applied to all of them
- If you put HTTP Header Manager as a child of a single HTTP Request sampler - it will be applied to this sampler only
Also according to the HTTP Header Manager documentation:
JMeter now supports multiple Header Managers. The header entries are merged to form the list for the sampler. If an entry to be merged matches an existing header name, it replaces the previous entry. This allows one to set up a default set of headers, and apply adjustments to particular samplers. Note that an empty value for a header does not remove an existing header, it justs replace its value.
Given proper placement of the HTTP Header Manager the headers defined there will be applied to the HTTP Request samplers residing under Test Fragments as well:
