Less room for manufacturer data than expected

Question

I created an App with one 16 bit service and some manufacturer data, but the maximum length of manufacturer data that doesn't cause an error is 16 bytes, when in theory it should be at least 20 bytes 
 If I construct the advertising data manually, I need 
 0x02,0x01,0x06 // Flags - 3 bytes taken for this
0x03,0x03,0x00,0x18 //Service ID (0x1800) - 4 bytes taken for this
0x17,0xff,......... ( 20 bytes of advertising data) - 22 bytes taken this
 
 However if I use ble_advertising_init what it sends to the SD is 
 0x03,0x19,0x00,0x00, 
0x02,0x01,0x06, 
0x03,0x03,0x00,0x18, 
0x13,0xff, (16 bytes of advertising data) 
 
 The difference is that ble_advertising_init() creates 
 0x03,0x19,0x00,0x00 
 
 at the start of the advertising data 
 Looking at the received data on my Android phone, The advertising data does not seem to contain this additional data. 
 Can someone tell me what this is, and why its being send to the SD (if its not actually being transmitted), because it appears to be wasting 4 bytes which I could use for additional manufacturer data 
 Thanks 
 Edit. 
 I cant find code 0x19 the BLE spec 
 www.bluetooth.com/.../generic-access-profile

RK · Accepted Answer

You have to check the bluetooth assigned numbers document (which the SIG could make a lot easier to find than they do) and you'll see that 0x19 is appearance. 
 So look at the code which makes the advertising data and see what you need to set for appearance in the structure to suppress it from being encoded. A cursory look at the latest version of the code I have here indicates you set the include_appearance flag to false.