There's been a lot of talk about Collections lately, especially with 2 activated concurrently:
Azure Lane (AL) and
In the Service of the Motherland
... so for those unfamiliar with some of the quirks of Collections, this is for you.
First, there are a couple of Categories of Collections ...
Permanent ... these are activated by completing Campaigns ... examples are Yamamoto Isoroku and American Cruisers (Hit Hard, Hit Fast, Hit Often)
Unavailable ... end of collaboration: High School Fleet
Events ... these include Collections created for and collected during special activities or promotions
Event Collections: after an event ends, collection items may:
continue to be available, but only if they are subscriptions (see below) or be
made available again through
Special missions ... e.g. current Saviors of Dunkirk and recent Bismarck in Action missions, or
Armory and/or Premium Shop ... e.g. Battle of North Cape,
otherwise they are
Inactive ... e.g. Belle Epoque, WG 3rd Anniversary
Collection Subscriptions: This is a special designation for Collections and applies to any collection that can be set to "Collecting" or "Don't Collect". The Permanent collections as well as AL and Motherland are Subscriptions. Only 1 Subscription Collection can be active (collecting) at a time. A Subscription Collection will always be available for you to Collect (as long as you have collected at least 1 item) and can drop items in any Daily Container.
Wiki - Collections
All About Collections
Sometimes WG will say a collection like Belle Epoque is "permanent". All I can assume is that maybe that means items will be made available again ... SoonTM
I hope this helps, but I don't claim to be an expert on this topic, so any corrections or additional info is welcome.
edit: Legion of Honor Subscription Collection added in Update 0.8.7, 21/Aug/19
edit: Royal Navy Destroyers and Naval Aviation added as Subscription Collections
edit: Great Eight added as Subscription Collection
edit: Updated Table Image 1/Sep/19
edit: Prelim Info added for Italian Cruisers & HSF Returns Collections, Updated Table Image 9/Nov/19