API 5L

"Through the Flowline" is actually related to Fishingtools and Downhole completions. Think of it more as a hydraulic line.

And what they mean by 'through the flow line' is this smaller line pipe is pushed down an already existing string... think of pushing hydraulic tubing with a piercer or plug down a water pipe.

"A flow line, used on a drilling rig, is a large diameter pipe (typically a section of casing) that is connected to the bell nipple (under the drill floor) and extends to the possum belly (on the mud tanks) and acts as a return line, (for the drilling fluid as it comes out of the hole), to the mud tanks."

through flowline (TFL)

1. adj. [Well Completions]Pertaining to treatments performed on subsea wells where the fluids and associated pump-down equipment, such as plugs or darts, are pumped through the flowline normally used for production fluids.

http://www.glossary.oilfield.slb.com/en/Terms/t/through_flowline_tfl.aspx

http://www.fedsteel.com/products/carbon-pipe-and-tube/api-5l-seamless-line-pipe.html

http://globaloilflow.com/beam-mounted-compressors/operating-range/