Start Process vs Start process smart service

HI [mention:1e619c45d5b545f7b7e926eeaf8d8cf7:e9ed411860ed4f2ba0265705b8793d05] 

Start Process and Start Process Smart service  both are same a!startProcess() is  used in expression  that can be executed in saveInto and smart service is used in the process model.

Start Process Smart service

They are the same smart service. One is called in rule and the other one is called in Process Model. 
Both will start your Process Model asynchronously. However, they are different from a!startProcessLink() which calls your process synchronously.

1. Using sub process, we can invoke a child process either synchronous or asynchronous as per our business requirement, whereas using start process, it always starts the target process asynchronous.

2. Because of above behaviour, if you have multiple tasks configured in various processes or their child processes, and you want to have chaining between the tasks, you need to go for sub process (synchronous with activity chain), where as if there is no need of having chaining in between these tasks then, we can go for start process.

3. An another use case: let's assume you have 2 tasks and the moment you submit task1, task2 should be visible due to chain, but before that we want some common operations to be perform (which is configured in a common process model), you need to go for sub process with synchronous approach, but if no dependency, they you can go for either sub process (asynchronous) or start process (recommend)

4. Using sub process, adds readability to understand, which process has been invoked while debugging whereas in case of start process we need to search for target process using its UUID (if the process UUID gets change based on provided input).

5. If we want to switch from one process to another along with its respective parameters, based on provided input, then it's preferable to go for start process instead of sub process

5. If you want to invoke a process from an Interface or Web API in such case you have only one option i.e. startProcess

6. And the most major difference, start process invokes the target process on a separate engine of Appian to Balance the load, hence we can't give guarantee, whenever we use start process, will immediately trigger the target process, whereas sub process gives you assurance of starting the child process immediately once after the flow hits this node.

https://community.appian.com/discussions/f/process/11614/what-is-the-difference-between-start-process-and-sub-process