The InPlace mates created automatically for in-context parts are there to prevent movement of the part. This is because the in-context part is attached to the geometry of parts in the assembly through external references, references that cross between parts at the assembly level. Changing the location of the part can cause changes to the geometry that may not be desired.
Replacing InPlace Mates You can remove InPlace mates using Delete, and remate the part using standard mate techniques. This technique gives you an option to leave a degree of freedom for movement. Generally, this works best if the face selected for the InPlace mate is perpendicular to the direction of motion, as this does not affect the part origin. Deleting InPlace Mates When you delete an InPlace mate, a warning message appears after the confirmation dialog.
The base sketch of the part located by the InPlace mate contains references to other entities in the assembly. These references may update in unexpected ways after this mate is deleted because the part will no longer be positioned relative to the assembly. Would you like to remove these references now? (No geometry will be deleted.)