Thinking of communications if you have variables that are spread about then it will take lots of send/receives to transfer the data, for example, many HMI manufacturers communications are optimised to reduce communication as much as possible so if you have a block of contiguous variables it can read/write them in one block, however, if your variables are spread about all over the variable database it would have to send/receive each variable separately, this increases the coms time as each time a variable is sent/received there will be a number of bytes on information like start/end and check sum chars so for example sending a single variable to a PLC will require perhaps a start char, an address char, variable type char, length char, the actual variable data then check sum & end char, so to send one variable may take 30 or so characters, whereas sending contiguous variable values only takes the same control & information chars plus the actual variables. would only increase this by the amount of actual variable data.
So in short, keep variables in blocks and bit variables in 16 bit boundaries, most HMI communication blocks can only transfer bits in 16 bit boundaries so if these are spread about over the variable bit address space it will be sending a lot of data that is not required.
This is particularly important in fixed address variables like Mitsubishi and many others where the variables are address based rather than symbol based.
