"What it is" is easier to explain than what it means.
LJ6C, often abbreviated to 6C, is a standard relating to the size of any communications or AV module, in our case, Cat5e, Cat6, Cat6a, HDMI, USB, & Audio Jacks. All LJ6C or 6C modules and inserts are 25mm wide and 38.5mm high but can vary in depth. Many experienced electricians or network engineers will associate LJ6C with floor boxes. LJ6C modules are used where faceplate or mounting space is lacking.
What it means:
An educated guess suggests to us that it's origins will be found deep in the history of Post Office owned British Telecommunications Plc. BT created and still use a part numbering system that includes many LJ references. LJ stands for "Line Jack". An example of a line jack can be found here 3/1A (correct reference LJ 3/1A - but nobody calls it that anymore, usually just referred to as a 3/1A or a BT 3/1A). The "C" most likely stands for "Connector" and so we can only assume that the 6 refers to size.
In Short, we are not 100% sure what LJ6C means or where it came from even after much trawling of the internet, but the above seems quite plausible, Please feel free to educate us if you know otherwise or can offer some correction.
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