It means it's an unsigned integer, to match the type of value.
Found from the web:
u8 unsigned byte (8 bits)
u16 unsigned word (16 bits)
u32 unsigned 32-bit value
u64 unsigned 64-bit value
s8 signed byte (8 bits)
s16 signed word (16 bits)
s32 signed 32-bit value
s64 signed 64-bit value
The type of an integer constant is the first of the corresponding list in which its value can be represented. Unsuffixed decimal: int, long int, unsigned long int; unsuffixed octal or hexadecimal: int, unsigned int, long int, unsigned long int; suffixed by the letter u or U: unsigned int, unsigned long int; suffixed by the letter l or L: long int, unsigned long int; suffixed by both the letters u or U and l or L: unsigned long int .
For Each ctrl As Control In Me.Controls("pnlMainPanel").Controls
If ctrl.GetType Is GetType(System.Windows.Forms.Panel) Then
For Each subCtrl As Control In ctrl.Controls
If subCtrl.GetType Is GetType(System.Windows.Forms.TextBox) Then
If subCtrl.GetType Is ...