The vast majority of generalized bridges out there today are monolithic. This means that they are tied to a specific verification mechanism that is implemented as a core part of the bridge construction.
Connext is the first example of a generalized crosschain messaging mechanism that is modular, plugging into the best available verification method for a given ecosystem.
A cluster is a sovereign set of domains that share security and can communicate with one another using trust-minimized methods. For example, Ethereum and its rollups form a cluster. Cosmos chains are another cluster. Singular "monolithic" chains, such as Solana are also their own cluster.
In the Message Verification section, we broke down the different methods that exist to secure a message that travels between two domains. As we noted, the best available mechanism for verifying messages is different based on the specific pair of domains we want to communicate between. Another way to say this is that there is a heterogenous topology for message verification mechanisms across all networks.
For example, between a rollup and its L1, the most secure verification mechanism is to use the rollup bridge itself.
The security of a domain is always the security of its weakest link. This means that any method of passing messages to a rollup that isn't the rollup bridge introduces at least some trust assumptions and security overhead, which in turn weakens the benefit of using a rollup (vs a less secure domain such as a sidechain) in the first place.
On the other hand, the most secure way to verify a message that passes between two discrete chains is a light client (zk or otherwise). Light client implementations exist in some places, but not everywhere yet - and it's highly unlikely that a single light client protocol will "win" every single pathway between sovereign chains.
Modular bridges make the verification layer (and potentially other parts!) of the bridging stack pluggable. By doing this, they can leverage existing methods of message verification wherever possible. This gives the best possible security for applications that may want to interact with multiple (heterogenous) domains simultaneously.