Originally, the doseNumber and seriesDoses elements in the immunization related resources were created with a data type of positiveInt (note that these elements have had different names in different base FHIR releases). This is probably because most doses can be expressed in the form of "dose 1 of 3", so having a number makes sense on the surface. But many vaccines are administered as "boosters" beyond the primary sequence of vaccines or "annually" such as influenza. For this reason, in R4, these elements were allowed to choose between positiveInt and string. But really, the fact that these concepts can be expressed as numbers, no "number type things" are ever done with the values (additional, order, etc). To simplify things, these elements should only be strings. 

The element descriptions should also be clarify that the source/intent of the elements in each resource is a bit different (that is, in Immunization, it's the intent of the provider in administering the vaccine and in ImmunizationEvaluation and ImmunizationRecommendation, its the outcome of the CDS engine evaluation of patient history against the specific set of recommendations it uses). 

See the Zulip thread for further details of the discussion: https://chat.fhir.org/#narrow/stream/211987-methodology/topic/ConceptReference