A. I'm not a licensed pro plumber.
B. I'm not from Canada.
C. I do not know plumbing code well.
D. I'm just an old DIYer.
E. I know of no reason that someone couldn't use a union in a non-pressurized drain line as long as it didn't leak.
F. Make an anonymous call to your local Building Inspection Department and ask if you want.
G. Go ahead and use the union.
Any joint in a drain line is where it will form a clog. Most piping under a sink has slip-joint connectors with friction type nuts or threaded connector. I have never seen an actual"union"in a drain pipe. Normally would be a Fernco or No-hub coupling.
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