Some questions on charsets

[23732]

Code:

C:\HashCat\oclHashcat-plus-0.08>oclHashcat-plus64.exe -a 3 -m 100 F:\RealWordlis
ts\SHA1\CleanedSHA1.txt -1 --custom-charset1=ab F:\RealWordlists\SHA1\Cracked.tx
t --remove
oclHashcat-plus v0.08 by atom starting...

Hashes: 2603924
Unique digests: 2603924
Bitmaps: 21 bits, 1048576 entries, 0x000fffff mask, 4194304 bytes
GPU-Loops: 64
GPU-Accel: 40
Password lengths range: 1 - 15
Platform: AMD compatible platform found
Watchdog: Temperature limit set to 90c
Device #1: Cayman, 2048MB, 0Mhz, 22MCU
Device #1: Allocating 132MB host-memory
Device #1: Kernel ./kernels/4098/m0100_a3.Cayman.64.kernel (443896 bytes)

ERROR: Mask is too big

Code:

C:\HashCat\oclHashcat-plus-0.08>oclHashcat-plus64.exe -a 3 -m 100 F:\RealWordlis
ts\SHA1\CleanedSHA1.txt -2 --custom-charset1=?d F:\RealWordlists\SHA1\Cracked.tx
t --remove
oclHashcat-plus v0.08 by atom starting...

Hashes: 2603924
Unique digests: 2603924
Bitmaps: 21 bits, 1048576 entries, 0x000fffff mask, 4194304 bytes
GPU-Loops: 64
GPU-Accel: 40
Password lengths range: 1 - 15
Platform: AMD compatible platform found
Watchdog: Temperature limit set to 90c
Device #1: Cayman, 2048MB, 0Mhz, 22MCU
Device #1: Allocating 132MB host-memory
Device #1: Kernel ./kernels/4098/m0100_a3.Cayman.64.kernel (443896 bytes)

ERROR: Mask is too big