use of series resistors

The various reference schematics are loaded with low value series resistors. With only a few exceptions, I’ve been unable to locate a source that explains when/where/value for these resistors. Can anyone help?

Recommended Placement and Sizing of Series Limiting Resistors to Control EMI Signal Value (OHMS) Comments BCLK 15 If external clock is needed. Place 15 ohm series resistor near BCLK pin. Optional External Oscillator 56 for NETA12-40 220 for NETA50 OE- 56 WE- 56 CS0* 56 CS0OE* NEEDS SERIES LIMITING RESISTOR NEAR NET+ARM ______ RAS1/CS1* 56 | RAS2/CS2* 56 | RAS3/CS3* | RAS4/CS4* | CAS0- 56 | CAS1 56 | CAS2- 56 |____ 56 Ohm Series limiting resistors are recommended CAS3- 56 | only if these are used as SDRAM control lines. BE0- (or (DQML) 56 | BE1- (or DQMH) 56 | BE2- 56 | BE3- 56 | _______| TXCLK NEEDS SERIES LIMITING RESISTOR NEAR PHY RXCLK NEEDS SERIES LIMITING RESISTOR NEAR PHY TXD0-3 56 NEAR NET+ARM RXD0-3 56 NEAR PHY Note: it can’t hurt to have a series limiting resistor on a pin. Some designers put them on all control lines to limit the emi from the instantaneous currents.