If the applicant files (a) missing part(s) (i.e. part(s) of the description, part(s) of the claims and/or part(s) or all of the drawings) or (a) missing element(s) (i.e. all of the description and/or all of the claims) which has (have) no basis in the priority document, the filing date of the application as a whole will be the date on which the part(s) or the element(s) was (were) subsequently furnished.
However, after the date of receipt of the purported international application, an applicant does have the possibility to furnish parts of the application or entire elements which were erroneously omitted without affecting the international filing date.
This can only be done before the RO within two months of the date of receipt of the purported international application (or at the invitation of the RO) provided that the priority claim was present at that initial date of receipt and only if the applicant can show that the missing part(s) or element(s) was (were) completely contained in the priority document. Missing parts or elements which have been accepted under this criterion are considered to be part of the application documents "as originally filed".
If the RO finds that the "completely contained" criterion is not met, the filing date of the application will be the date on which the part(s) or the element(s) was (were) subsequently furnished (unless, in the case of missing parts, the applicant withdraws the subsequently furnished parts). Where the EPO is ISA or IPEA, the examiner must check (as far as the documents needed are available) whether the RO's assessment of the "completely contained" criterion was correct.
See also GL/PCT-EPO A‑II, 5.