Sensor-based messages are contained within the directory:
Defining Custom Messages
ROS messages are defined in .msg files.
An automatic MD5 sum of the .msg file is created and is checked when nodes try and publish/subscribe to topics. Nodes can only communicate when both the message type and MD5 sums match.
ROS messages may include a special header. This can contain a time stamp and frame ID.
If the message types are not the same, you will normally get a runtime error on launch similar to:
[ERROR] [1481703140.912974056, 0.100000000]: Client [<node_name>] wants topic <topic_name> to have datatype/md5sum [<path>/cd5e73d190d741a2f92e81eda573aca7], but our version has [<path>/2d3a8cd499b9b4a0249fb98fd05cfa48]. Dropping connection.