This message is created using a very rigid format the + and : are field separators and the ' the end of line marker.
The + separates fields that must be present even if they have no values, hence the long string of them, this is necessary as all fields are defined by their position.
The : separates fields that are optional.
The MHD segment says what the message is, in this case the 1 is a sequence number of the message in the file, PROP05 means a new business message and the 2 is the variant of that message.
The PYD segment is payment details and each position's meaning is shown in the table.
Just to stress the point the meaning of a value is entirely depended upon where it appears, so both sides the sender and the receiver agree to this outside of the actual message.
Not only do both parties have to agree the position they also have to agree the format, for example PPCB is S9(8)V99.
You may notice that the fields from RTPA don't appear in the message above, it is allowed to end a segment early and all missing elements are assumed to be empty.
STX= - Start of Transmission
MHD= - Message Header
CID= - Communicators ID
SFI= - Software Identifier
SFT= - Software Identifier Trailer (count of SFIs)
PLH= - Policy Header
PYD= - Payment Details
A full message consists of a lot more segments laid out like those shown.