2 Key Difference between PER_ALL_ASSIGNMENTS_M and PER_ALL_ASSIGNMENTS_F in HCM Cloud

• It stores two levels of the 3-Tier Model: Employment aka Placement Terms (Level 2) and Assignments (Level 3).
• The assignment type is used to differentiate between these two levels
• Differentiate among employee, contingent worker, applicants, and benefits assignments.
• This is date-tracked and allows multiple changes in a day.
• Primary key of this table is ASSIGNMENT_ID

PER_ALL_ASSIGNMENTS_M is a TABLE

PER_ALL_ASSIGNMENTS_F is a view based on PER_ALL_ASSIGNMENTS_M table. This view only shows person assignments whose latest change is effective.

SELECT *
FROM
  PER_ALL_ASSIGNMENTS_M paa
WHERE
  EFFECTIVE_LATEST_CHANGE = 'Y';

So the answer to question, when to use PER_ALL_ASSIGNMENTS_M  table is that if you want all the data use the base table. 

And,  If you want the effective latest change only use PER_ALL_ASSIGNMENTS_F

If there are changes to employee assignment in a day you will see multiple records in the PER_ALL_ASSIGNMENTS_M table.

per_all_assignments_m_ is another table which is the audit table. It stores the history of changes to assignment objects when auditing is enabled. (as a convention all table ending with _ are audit tables)

SQL query to fetch Employee in fusion hcm

The assignment table is joined to per_all_people_f the master table for employee records and org tables as per below sample query.

SELECT perf.person_number, hruf.name businessunit
  FROM hr_org_unit_classifications_f hrc,
       hr_all_organization_units_f hrall,
       hr_organization_units_f_tl hruf,
       per_all_assignments_m pasm,
       per_all_people_f perf
 WHERE hrall.organization_id = hrc.organization_id
       AND hrall.organization_id = hruf.organization_id
       AND hrall.effective_start_date BETWEEN hrc.effective_start_date
       AND hrc.effective_end_date
       AND hruf.language = 'US'
       AND hruf.effective_start_date = hrall.effective_start_date
       AND hruf.effective_end_date = hrall.effective_end_date
       AND SYSDATE BETWEEN hruf.effective_start_date
       AND hruf.effective_end_date
       AND hruf.organization_id = pasm.business_unit_id
       AND pasm.person_id = perf.person_id
       AND pasm.assignment_type = 'E'
       AND pasm.effective_latest_change = 'Y'
       AND SYSDATE BETWEEN pasm.effective_start_date
       AND pasm.effective_end_date
       AND perf.person_number = NVL (:personnumber, perf.person_number)

FAQs

Suggested reading HCM Cloud ATOM Feeds

References : Oracle doc on HCM Tables