9.1.3   Colon used Block:Parameter ( : )

The colon ( : ) appended to the Block Name, followed by the Parameter name, to read parameter of a Block in the Point Browser, Trends, and Graphic Displays.

Parameters are the elements in a Block

Users and operators will referenced the elements in a block by appending the parameter name to the Block Tag name: Blockname:parameter.  You should pick a parameter name that will be meaningful to ordinary users and operators. 

For example, a Foxboro 761 device:

TIC101:SP for the setpoint of a Temperature controller,
TIC101:MEAS for the measurement,
TIC101:OUTP for the output of the primary controller
TIC101:OUTS for the output of the secondary controller
TIC101:AM for the auto manual state
TIC101:RL for the remote local state
TIC101:PB for the proportional band tuning constant
TIC101:INT for the integral tuning constant
There are probably 50 more parameters in a Foxboro 761 controller.

The real working creating a block is to create the parameters that will be grouped into the block. Once you create a block, you cannot change the parameter name without re-creating both the block-type and the block. 

Notice that the parameter names are limited to 7 characters.  Also, parameter names can be used again in a different device type (for example you could have a SP parameter in all device types).  But a parameter name must be unique within a device type.