Its by value, but most things internally are references. For example, a Modbus master has a TCP ref and a lock (to stop you from accessing the connection on multiple threads at the same time) and the master has a data model (which contains DVRs of all of the registers, coils, etc which constitute a slave). A few things are values, like which slave ID the master is talking to (handy on multi-drop serial or tcp-to-serial networks).
I generally wouldn't recommend sharing a master between parallel loops. Because modbus is a request-response protocol you can only handle a single request at a time on a given connection (hence the lock inside to prevent accidentally breaking this rule). Its also just strange to me why you'd want to talk to a single slave from multiple loops, I would normally write some sort of intermediary driver to hide how terrible modbus is from the rest of the application (ie you send your modbus loop "please set pressure to 25" and the solitary modbus loop converts this sane and reasonable request into "Write holding registers, 0-2, (uint[])25".
If you really do need to have multiple loops talk to one slave:
1-If TCP, just open multiple separate connections. This gets rid of the lock and most slaves will support it
2-If serial, you're more likely to run into issues because of how visa works. You could technically make two masters on the same visa port, but visa won't protect you from accidentally sending two requests simultaneously, which could lead to reading back the responses in the wrong order, which is of course not great. For this, I'd recommend using your own DVR or global variable (have one loop be the 'owner' who has to maintain the connection, but other loops can grab it and attempt to write to it, ignoring any errors or perhaps sending a "hey do your job" to the loop who is supposed to maintain it).
