# Understanding commands

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Commands are messages that represent intent, commands can be rejected in distributed systems.

In EventCatalog [Services](/docs/development/guides/services/introduction.md) may invoke (send) or accept (receive) commands in your architecture.

### Example of a command[​](#example-of-a-command "Direct link to Example of a command")

An example of a command would be `PlaceOrder` message over HTTP.

* This message is used to place an order in a system
* Commands can be rejected, the `PlaceOrder` may be rejected by the service that processes it.

### Commands in EventCatalog[​](#commands-in-eventcatalog "Direct link to Commands in EventCatalog")

* Commands in EventCatalog can be **accepted** by services or **invoked** by services.
* Commands in EventCatalog are blue (following [EventStorming conventions](https://www.eventstorming.com/))
* Commands live in the `/commands` folder.