RAI Manual page 2-46 explains this and more, "Completed when a resident whose SNF stay was being reimbursed by Medicare Part A is hospitalized, discharged return anticipated, and then returns to the SNF from the hospital within 30 days and continues to require and receive Part A SNF-level care services. Under these conditions, the entry tracking record completed upon return to the SNF will be coded as a reentry with Item A1700 = 2."
In other words, a 5-day follows an admission for a Medicare resident while a resident who discharged - return anticipated while on Medicare and returns on Medicare gets a readm/return assessment instead of a 5-day.
An entry tracking record is required EVERY time a resident is admitted or returns after discharge. OBRA/PPS/OMRA assessments are in addition to the entry. At the bare minimum, a resident who enters and leaves the facility would receive an entry and a discharge. All other assessments are performed as needed.