Using RIGHT MAX CHARINDEX to get value

Question

declare @lotTech varchar(64)
declare @lontNum varchar(64)SELECT @lotTech = RIGHT(MAX(l.lotID),CHARINDEX(' ','TECH'))
FROM lots l
if (@lotTech = '')
SET @lotNum = 'TECH 1';
else
SET @lotNum = 'TECH' + SPACE(1) + @lotTech + 1;
Can't get this to find if there is a null value or max value and 1 to concatnated data? 

Answer 1

@productID as bigint,
@lotNum as varchar(64),
@manufactureDate as datetime,
@expirationDate as datetime = null,
@gradeDesc as varchar(64)
as
BEGIN
declare @lotcheck varchar(64)
declare @lotTech varchar(64)
SELECT @lotcheck = lotid FROM Lots WHERE lotid = @lotNum
if (@expirationDate is null)
begin
SELECT @expirationDate = dateadd(day, ch.expiration, @manufactureDate)
FROM products p
INNER JOIN chemicals ch on p.chemicalID = ch.chemicalID
WHERE p.productID = @productID
end
if (@gradeDesc = 'TECH GRADE')
begin
SELECT @lotTech = RIGHT(MAX(l.lotID),CHARINDEX(' ','TECH'))
FROM lots l
if (@lotTech = '')
SET @lotNum = 'TECH 1';
else
SET @lotNum = 'TECH' + SPACE(1) + @lotTech + 1;
end
if (@lotcheck is null)
begin
INSERT INTO Lots(lotID, chemicalID, expirationDate, manufactureDate, confirmed, productID)
SELECT @lotNum as lotID, p.chemicalID, @expirationDate as expirationDate, @manufactureDate as manufactureDate, 0 as confirmed, p.productID
FROM products p
INNER JOIN chemicals ch on p.chemicalID = ch.chemicalID
WHERE p.productID = @productID
end
else
begin
UPDATE Lots
SET manufactureDate = @manufactureDate,
expirationDate = @expirationDate,
productID = @productID
WHERE lotID = @lotNum
end
return
END
I'm showing more of the TSQL which works fine but the part I'm not understanding is the use of essentially parsing the data that is TECH and the number which has to be the maximum number for TECH.
The output should be an ascending value concatnated with the word 'TECH' and a space.  Initially there is nothing in the lotNum column.  I need to get the MAX number out of the lots table for the paticular grade descripton -  'TECH GRADE'
and then add 1 to that retrived number for the insert of the record. 
So the first record to be inserted after the MAX number is found to be not present is
TECH 1
Following queries have to find the MAX number in the whole table and then add 1  and insert when 'TECH GRADE' description so
TECH 2
TECH 3