hi

1)Answer:
selection screen validations are doing in At selection
screen  on field event. In this event u can write ur screen
validations.

Eg:

Select-option: carrid1 for sflight-carrid.

At Selection-screen.
if carrid1-low = '' and carrid1-high='';
 error msg
endif

In above example, if u did not enter any values for carrid1,
then it displays error message

In module pool programming,validations are doing in PAI event 

example:
Field sflight-carrid values('LH')

In this example, u must enter value LH, other wise the
screen is not moving.


2)Answer: 

Search help means, it displays a list of all possible values
for screen field when u press F4 key .

Search helps are 2 types, Collective and Elementary search
helps.
# Elementary search helps describe a search path. The
elementary search help must define where the data of the hit
list should be read from (selection method), how the
exchange of values between the screen template and selection
method is implemented (interface of the search help) and how
the online input help should be defined (online behavior of
the search help).

Collective search helps combine several elementary search
helps. A collective search help thus can offer several
alternative search paths.