OK, we just about gave that away in the prelude to the questions. It's flexible
metal conduit. It's made by helically (think spiral) winding metal strips in a
way that causes them to interlock. These are special, pre-shaped strips formed
specifically for this process.
348 lists only one permitted use [348.10]. It can be used "in exposed and
concealed locations." That's quite vague. Does this mean you can use it
underwater? No. You can use it except as prohibited under "Uses Not Permitted."
So you can generate a list of 100 permitted uses if you have the time to sit
down and write it out.
From an engineering standpoint, you want to avoid using FMC except where you
need the flexibility. It's necessarily weaker than RMC or IMC and is more easily
broached due to its continuous joints. Don't use it for the reason of trying to
get around raceway support issues. It still must be supported, and it's going to
be more difficult to install than RMC or IMC simply because it is flexible.
lists seven prohibited uses [348.12]. The last one "where subject to physical
damage" is the standard "prohibited use" for raceway; think of it as a
disclaimer for violations of common sense. The others are also derived from
common sense, and are based on the fact the joints of the flexible conduit are
not the same as the walls; things can get in, rather easily.
If you use
angle connectors, they can't be concealed [348.42]. As a general practice, use
FMC as short-length means of connecting IMC or FMC to the final load. For
example, you need to connect a motor but you don't want to vibrate your conduit
off the wall. Use FMC between them.
Your thought process in deciding where to use is essentially that of extending
an existing RMC or IMC run, but with flexibility added to accomplish a specific
goal. Such as connecting to a motor. Note that it's no accident that FMC comes
in sequence after RMC and IMC.
bushings, couplings, connectors, brackets, etc., listed for use with FMC.
Article 348 doesn't explicitly state this, but the listing requirement is a
general requirement provided elsewhere in the Code.
You can't, for example, use
connectors designed for use with plumbing systems. Nor can you run a length of
PVC in an FMC system. You can connect a PVC system to an FMC system, but that
would be an atypical setup. That setup would also require running a bonding
jumper through the PVC to eliminate differences of potential along the FMC.
Generally, don't mix metallic and nonmetallic raceway.