The top entity in the BidCore hierarchy is an Advertiser. The Advertiser is the parent entity for Campaigns and for assets such as Creatives and tracking Pixels. A Campaign, in its turn, is the parent entity for the Line Items associated with it.
Trading or targeting entities such as an SSP or Deals exist independently of the Advertisers and Campaigns. Likewise, targeting options such as postcode or app bundle lists exist independently of the Advertiser too and can be used by entities associated with all Advertisers.
Other items are specific to the Advertiser entity, such as if you create an asset (e.g. a tracking Pixel) under a certain Advertiser account, you cannot reuse it with any other Advertisers. If you need to use a Pixel with the same settings across several Advertisers, you need to create an appropriately associated Pixel for each of them.
Whenever you delete a parent object, all of the associated child objects are deleted. So if you delete a Campaign all its associated Line Items are deleted, and if you delete an Advertiser, all of its associated Campaigns, Line Items, and pixels are deleted.
It is strongly recommended that you do not delete parent entities unless you are absolutely sure that you can do it safely without unintended consequences. Likewise, whenever an entity is deactivated and paused, all trading child entities are deactivated and paused too.