about 802.11 scan and probe request

dear all,
  is there anyone who read the standard "Part 11: Wireless LAN Medium
Access Control (MAC) and Physical Layer (PHY) Specifications" ?
  and i spend two month to read the stardard, and i read a few code of 
wpa_suppliant and
wpa_gui,there is one question: the scan have two mode:active or 
passive,but when i run
the wpa_gui, there is only scan button, in the standard the scan 
request is like
MLME-SCAN.request (
                BSSType,
                BSSID,
                SSID,
                ScanType,
                ProbeDelay,
                ChannelList,
                MinChannelTime,
                MaxChannelTime
                )
but there are only three MAC frame formats:data frame, control frame 
and Management
frame, and there is no scan frame, there is only a Probe Request frame, 
    so i am
stumped, what's the relationship between MLME-SCAN.request and Probe 
Request frame, is
the MLME-SCAN.request and Probe Request frame only in different layer? 
but the items is
also different, the MLME-SCAN.request have:(
BSSType, BSSID, SSID,  ScanType, ProbeDelay,  ChannelList,  
MinChannelTime,   MaxChannelTime), but the  Probe Request frame body