My understanding of the notation here is:
1) N:1 parent records are listed by display name of the lookup field (the singular name of the target entity by default, but often customized for clarity, or where multiple relationships exist eg to systemuser). “Name of lookup field” eg on Contact: “Originating Lead” is a lookup field, and used in the relationship list too.
1.a) Some lookups are listed twice or more for “polymorphic” relationships such as Customer or Regarding displayed multiple times as “Name of lookup field (target entity display name)” for example on Opportunity we see “Potential Customer (Account)” and “Potential Customer (Contact)”. Note the singular form of the entity name. On Contact “Company Name (Account)” means the one parent Account of this Contact (note that Company Name is simply the new display name for Parent Customer field)..
Some N:1 show the entity name even though they are not polymorphic eg “Created By (User)”
2) 1:N child records are listed by plural display name of related entity. The plural name reinforces the nature of the one-to-many relationship (for those who know to look for this). Lookup field is always shown in parentheses. “Child Entity Plural Name (name of lookup field on Child Entity that refers to the current entity)” for example “Accounts (Primary Contact)” (note the plural name) means all Accounts which have this Contact as their Primary Contact
∴ in (1) and (2) Words in parentheses can refer to the name of a polymorphic N:1 target entity, or the lookup field on a related 1:N child entity. The same syntax represents two very different things.
3) N:N associated entities show just “Plural name of entity” from the other side of the relationship (not the hidden intersect). Eg on User we see “Teams” (as well as lots of other “Team” entries), which is the Teams the User is a member of. On Teams we see “Users”.
I think this syntax could definitely be improved. Especially 1a could use square brackets rather than parentheses for example, to separate this from use case 2.
Your examples work out like this:
Business Breakdown - you have a lookup field to a single Business Breakdown record
Business Breakdown (Account) - you also have Business Breakdown as a child record (1. Maybe you have this two ways in order to create a "1:1" relationship. 2. Does it not read "Business Breakdowns (Account)"? Unless the plural name of that entity is wrong 3. Consider making one or other of these relationships not searchable, to reduce confusion)
Leads - N:N relationship with Leads for this Account
Leads (Customer) - child Leads where this Account is specified as the Account in the "Customer" lookup on the Lead
Leads (Parent Account for lead) - child Leads where this Account is specified as the Account in the "Parent Account for Lead" lookup on the Lead
