When you diagnose (dx) a patient during an encounter you may notice that sometimes that diagnosis is automatically moved into the patient's PMH block, or that a suggestion appears prompting you to add that diagnosis to their PMH block. For certain diagnoses (Hypertension, Obesity, etc) it is very likely that these should belong on the master PMH list, whereas acute issues (lacerations, strep throat, infections) probably do not belong on that list. These actions are controlled on a per-ICD10/diagnosic code basis under the Diagnostic Codes manager under the Admin > Manage > Diagnostic Codes menu item.
To manage how specific codes behave, just open that Diagnostic Codes manager, find the code you want to edit, and then select how it should behave under the PMH Action drop-down:
Also note that when you're looking at the Diagnostic Codes manager, you can see the current PMH action for any given diagnostic code by looking at the right-most column:
If it has a grey background, then no action will be taken when being diagnosed, an orange background indicates that it will prompt the user to add it to the Past Med Dx block when it is diagnosed, and red indicates that it will automatically move onto the PMH list when it is diagnosed without any user prompting. Note that changing the PMH Action status of a diagnosis will not retroactively change any patients' past-medical-history entries.