Strange JOIN ON clause in T-SQL
GLFiscalYearPeriods is a table and this comma implies a cross join. (Cartesian product)

It seems that the query returns some values for each Fiscal Year.

Given this tables:

create table a (id int, foo int);
create table c (id int);
insert into a values (1,1),(2,2),(3,3);
insert into c values (10),(20);

select * from a, c;

select * from a cross join c;




id | foo | id
-: | --: | -:
 1 |   1 | 10
 2 |   2 | 10
 3 |   3 | 10
 1 |   1 | 20
 2 |   2 | 20
 3 |   3 | 20



db&lt;>fiddle here

Just another example:

create table a (id int, foo int);
create table b (id int, foo int);
create table c (id int);
create table d (id int);
insert into a values (1,1),(2,2),(3,3);
insert into b values (1,1),(2,2),(3,3);
insert into c values (10),(20);
insert into d values (1),(2);




select * from a join b on a.id = b.id       ,     c join d on d.id = a.id ;



  Msg 4104 Level 16 State 1 Line 1
  The multi-part identifier "a.id" could not be bound.


but if you cross-join it:

select * from a join b on a.id = b.id  cross join  c join d on d.id = a.id ;




id | foo | id | foo | id | id
-: | --: | -: | --: | -: | -:
 1 |   1 |  1 |   1 | 10 |  1
 1 |   1 |  1 |   1 | 20 |  1
 2 |   2 |  2 |   2 | 10 |  2
 2 |   2 |  2 |   2 | 20 |  2



db&lt;>fiddle here