What is the difference between % FS and % Reading?

There is a difference: 

ASM1300PG2M1103A00  the "A" indicates the accuracy is ±0.05%FS (±0.15 psi). This means that no data point will be off greater than 0.15 psi. 

ASM1300PG2M1103B00  the "B" indicates the accuracy is ±0.1% Reading. Results will be better in all pressures < 150 psi 

Error @ 0.05%FS        Pressure        Error at 0.1% Reading 

0.15 psi               30 psi                0.03 psi 

0.15 psi              100 psi            0.10 psi 

0.15 psi              150 psi            0.15 psi 

0.15 psi              200 psi            0.20 psi 

0.15 psi              300 psi            0.30 psi 

Either method will have a Total Error Band < ±0.25% (±0.75 psi)  

TEB = % linearity + % hysteresis + % non-repeatability  + span & zero settings + thermals