Heh, it's what Hans is best known for (the 555)... but in CML flavor instead! (Current mode logic -- see if you can spot the three diff pairs on the right side, and the positive feedback making it, in fact, two comparators and an R-S flip-flop!)
"SUB" is just substrate (common). For typical fabrication (or at least, contemporary), as long as this is the lowest potential in the circuit, no substrate diodes get forward-biased, and everything will behave as shown.
It looks like Q11 is intended to literally have double emitters. The current through Rext feeds the Q6-Q11 current mirrors. One of which feeds Q1-Q5, another mirror (probably only as complicated as it is, because of poor lateral PNP, and a relatively high desired accuracy). So when Q13-Q14 steer the current from Q11 (via Q8) into the capacitor, it discharges, but this must be at least a little greater than the (not switched) pull-up current (from Q5, etc.). For ~50% duty cycle, they should be equal and opposite when summed together. So, Q11 must draw twice the current, or, literally, have twice the emitter area.
Discrete transistors make crappy mirrors, so you want to use emitter resistors (on the ones touching 0V or +V), and ideally, use a programming current such that the voltage drop on those resistors is a reasonable fraction of Vbe, maybe 0.1-0.2V at the maximum intended current. At lower currents, the mismatch (due to manufacture and temperature differences in the transistors) will be greater, leading to e.g. drifty frequency and duty cycle, and poorly balanced duty cycle. (Duty cycle already will be quite wonky for low Rext currents (high values), due to base current drawn by Q16. This can be addressed by adding a JFET buffer between Cext and Q16.)
RBM6 being as small as it is, means the square wave output will be minuscule. This also means it'll be as fast as absolutely possible (essentially no loading due to Q14 collector capacitance, Miller effect), but that also means your signal is probably a small fraction compared to any residual +V ripple or noise -- use it carefully! Presumably, it can be made as large as R5/R6, with some cost to speed by then.
Tim