From MPEP documentation I am not understanding the difference between forms for 37 CFR 1.136(a) vs. 37 CFR 1.136(b). The former clearly states the relevant fees; the other does not. The latter one (b) provides (actually requires) the option to write justifications for the extension. But I don't find documentation distinguishing the purpose of one versus the other – at least not that I understand. Are there are guidelines, or known example histories, for what kinds of reasons could be accepted in 37 CFR 1.136(b)?
The text of 1.136(a) says it covers extensions to reply deadlines -
(i) Applicant is notified otherwise in an Office action; (ii) The reply is a reply brief submitted pursuant to § 41.41 of this title; (iii) The reply is a request for an oral hearing submitted pursuant to § 41.47(a) of this title; (iv) The reply is to a decision by the Patent Trial and Appeal Board pursuant to § 41.50 or § 41.52 of this chapter or to § 90.3 of this chapter; or (v) The application is involved in a contested case (§ 41.101(a) of this title) or a derivation proceeding (§ 42.4(b) of this title).
The case of responding to a run of the mill OA (rejection, objection, restriction requirement) is not on that list of exceptions so 37 CFR 1.136 (a) would apply to the situations you are talking about.
No reason is needed and it can be invoked in a blanket manner before examination even starts -
written request may be submitted in an application that is an authorization to treat any concurrent or future reply, requiring a petition for an extension of time under this paragraph for its timely submission, as incorporating a petition for extension of time
The fees must be paid. Not mentioned in this section is the mechanism of a blanket authorization covering an application that any missing or inadequate fee payment can be taken from a USPTO deposit account. If both these blanket statements are in place then the situations covered by 1.136(a) do not need any explicit petitions for the extension to happen automatically.
This type of extension only gets you to the statuary deadline. After that there is no petition to extend. Your application goes abandoned and then the relevant petition is a petition to revive. That is covered by 37 CFR 1.137. Its criteria is unintentional delay.
The USPTO is relatively loose on revival. They do not do any investigation on the affidavits submitted to claim unintentionality. If push comes to shove and the patent is enforced, the defendant will see the record and see the possible weak link to attack to try to invalidate on the grounds that the revival was granted on false or inadequate reasons.