Institutional ones: certified transcripts and degree confirmation, professional standing letters, any foreign credential evaluation, and a citizenship document. These set the schedule for everything else.
Request what other people control
The distinction that matters when ordering the work is between documents you can produce yourself and documents somebody else has to issue. A duty description, an organisation chart or a covering letter can be written in an afternoon whenever it is needed. A registrar's certified record, a professional body's letter of standing and an evaluation of a foreign degree cannot, and any of them can take weeks or come back with a discrepancy that needs resolving. Request every item in the second group before drafting anything in the first. It also helps to ask each issuer what name and wording will appear, since a document naming a discontinued programme or an earlier legal name is better addressed while there is still time.
At the first review, make a request log that names the issuer, the fact required, and the backup record if the original arrives late. A registrar’s sealed transcript settles education; a licensing body’s current letter settles standing; a passport settles nationality. Compare names, dates, and credential titles across those records before drafting the employer letter. A mismatch is not always disqualifying, but it needs a contemporaneous explanation. The decision is whether the file can be assembled from available records or whether a correction must be sought first. Hypothetical example: a veterinary pathologist finds that her diploma uses a former surname, so she obtains the legal name-change record before asking the prospective employer to prepare its support letter.