T/R SWITCH QUESTIONS (BIASING & U-LINE)

Using PIN diodes is new to me. I've read a lot of the app notes from various vendors like MA/COM, AVAGO, MICROSEMI. My problem is I don't understand how to bias the pin diodes. All of the app notes shows the rf path with the pin diode "isolated" with dc blocking caps. Voltage goes through an inductor, the pin diode, and then another inductor. No problems yet. But:

What limits the current?

Also, someone sent me a T/R switch sketch. It has some "U-LINE" depictions.

What is a U-LINE?

Thx.

Generaly speaking,PIN diode is current driving device. so the best way to bias a PIN diode is by current source. However since the on resistenc is small, you canalso bias it by a voltage source followed by a large seriese resister, this almost eaqul to a current source under samll bias cueernt.