What are the variations in resistance?

Figure 1 Basic BRT circuit
Figure 1 Basic BRT circuit

There is ±30% variation in the value of the series bias resistor (R1) from the typical value as shown in the datasheet. The value of the base-emitter resistor (R2) is not specified separately; instead, the resistor ratio (R1/R2) is specified. There is ±10% variation in R1/R2 from the typical value.
The values of bias resistors are dependent on temperature, as shown in Figure 2. The resistance decreases at a rate of roughly 0.2%/°C.

About resistor R1:
R1 converts the voltage applied to the B terminal of a BRT into current. A bipolar transistor is a current-driven device. When a bipolar transistor is voltage-driven, the rate of change of the collector current with respect to voltage becomes large, making it difficult to control the collector current. R1 in a BRT makes it relatively easier to control the collector current. When a BRT is on, the internal transistor operates in the saturation region where hFE (=IC/Ib) is in the range of 10 to 20, depending on the input voltage. Therefore, a relatively large current (IB) on the order of a few milliamperes flows through R1. Since the allowable power dissipation of R1 is 1/8 W, the maximum input voltage (VI) of BRTs with a high R1 value is determined by the value of R1.
(See the FAQ entry “What is the maximum voltage that can be applied to the base of a bias resistor built-in transistor (BRT)?”.)

About the resistor ratio (R1/R2)
The input voltage (ON) specification (VI(ON)) is dependent on R1/R2. Since the base current (IB) does not flow immediately before the transistor turns on, the voltage applied to the B terminal (VI) is divided by R1 and R2. Let the turn-on threshold voltage of the internal transistor be Vbe. Then,    
Vbe = R2 / (R1+R2) * VI(ON)
The Vbe of BRTs is not affected by the values of R1 and R2 if they contain the same transistor.
VI(ON)=Vbe*(R1+R2) / R2 = Vbe*( 1 + R1 / R2 )
Hence, VI(ON) is dependent on the resistor ratio (R1/R2).

Figure 2 Built-in Resistance-vs-Ta curve
Figure 2 Built-in Resistance-vs-Ta curve