Strange JOIN ON clause in T-SQL

I am untangling some legacy code and never in all my years have I seen this:

Select * 

FROM GLAccounts
INNER JOIN GLCharts
    ON glaGLChartID = glcGLChartID
LEFT JOIN GLCategories
    ON glcGLCategoryID = gltGLCategoryID
INNER JOIN GLDepartments
    ON glaGLDepartmentID = gldGLDepartmentID
INNER JOIN GLDivisions
    ON glaGLDivisionID = glvGLDivisionID
        ,GLFiscalYearPeriods                --
INNER JOIN GLFiscalYears
    ON glfGLFiscalYearID = glzGLFiscalYearID

ORDER BY glcGLCategoryID
    ,glcParentGLChartID
    ,glaGLChartID

the join works, but I don't find this use of a comma in any T-SQL syntax reference for the ON clause.

What's happening here?
TIA

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<>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<>fiddle here

StackExchange Database Administrators Q#260768, answer score: 21