Categories of Blockchain

Generally speaking, there are three categories of blockchain-like database applications; public, private and consortium

Public: anyone read and submit transactions, and anyone can participate in the concensus process. These platforms are secured by mechanisms such as POW (proof of work) or POS (proof of stake).

Consortium: concensus is controlled by a preselected set of nodes and rules for achieving concensus. The right to read the blockchain can be open to the public or can be restricted to a known set of participants. For example, 10 banks in consortium that agree to the concensus rule that 7 of 10 banks must sign (approve) a block for it to be considered valid representation of the truth

Private: Write permissions are kept centralized to a single organization or part of it. Read permissions may be public or restricted to a set of known participants.

Here is a list of the advantages of a private blockchain:
• When read permission is enforced, a private blockchain provides privacy.
• A consortium or private entity can change the rules of a blockchain, allowing them to revert transactions, modify (fix) balances, etc. This functionality may be deemed necessary, although we must say these reasons look more like patches to fix defective functionality.
• The miners (or validators) are identified, defined, and known, so any risk of a 51 percent attack from colluding bad actors does not exist or can easily be identified and dealt with.
• Transactions are less expensive, since they only need to be verified by a limited set of trusted (and identified) nodes instead of by every node on the public network. There does not have to be any transaction fee. There may still be a requirement to run a validator node to participate in the network. This would be an expense, but would be less expensive to participants than a transaction fee on the public network. The validator node may also provide protection against a distributed denial-of-service (DDoS) attack.
• With fewer miners needed, consensus finality can be achieved much faster among a smaller group.

Private Blockchain Use Cases
The public blockchain has, in many cases by design, the following challenges to enterprise
adoption, particularly those industries that are highly regulated:
• No privacy
• Anonymous processors
• Little to no governance
• Limited throughput with slow transaction confirmation (performance challenge)

In short, there are essentially two aspects to privacy itself: read permission and block generation (or write) permission. If the use case requires privacy, you will want to restrict read permission. If the use case requires trust spread across a limited and predetermined set of participants, you will want to restrict write permission. Both aspects individually or collectively may push you to a private (or consortium) blockchain platform.