Need to replace case statement in where clause - tsql

Question

**The below query is working badly, and I'm planning to replace case stmt in where clause with temp table. Pls suggest to me some option to rewrite this query
Is there a way to simplify this into multiple SQLs
Also explain me how this case stmt works in where clause**
select DISTINCT
ph.order_type_cd,
c.order_num,
ph.created_date,
os.order_status_desc,
ph.facility_name,
PH.vendor_order_num,
l.lic_nm,
(u.last_nm + ', ' + u.first_nm )as requestor_nm,
bl.billing_location_desc,
ph.ship_to_name,
dm.dispatch_method_name_desc as courier,
PH.po_last_updated_dt as last_updated_date,
(u1.last_nm + ', ' + u1.first_nm )as last_updated_by,
ph.right_grp_id,
c.line_id,
a.material_product_id,
a.parent_material_product_id
from avt_po_detail c
inner join avt_po_hdr ph on ph.order_num = c.order_num
left outer join avt_material_product AS a on a.material_product_id=c.material_product_id
INNER JOIN #entity_ids po on po.entity_id = c.order_num --or #select_pos = 0 )
LEFT OUTER JOIN dbo.ert_product AS pc ON pc.product_id = a.product_id
left outer join ert_product_type as pt on pt.product_type_cd = pc.product_type_cd
LEFT OUTER JOIN dbo.ert_product_xref as xref on xref.product_id = a.parent_product_id AND xref.version_id = 1
left outer join avt_order_status AS OS ON OS.order_status_cd = PH.order_status_cd
left outer join avt_licensee l on l.lic_id = ph.lic_id
left outer join avt_po_from pf on pf.material_product_id = a.material_product_id and pf.line_id = c.line_id
left outer join avt_po_audio_detail pad on pad.line_id = c.line_id
left outer join avt_materials_trans_material MTM on mtm.sequence_no = c.video_seq_no
left outer join avt_user u on u.userid = ph.requestor_id
left outer join avt_user u1 on u1.userid = ph.po_last_updated_by
left outer join avt_billing_location bl on bl.billing_location_cd = PH.billing_location_cd
left outer join avt_dispatch_method_name dm on dm.dispatch_method_name_id = PH.dispatch_method_name_id
left outer join avt_right_group arg on arg.right_grp_id = ph.right_grp_id
left outer join #prod product on product.product_id=a.product_id and (product.version_id=c.dist_version_id OR product.version_id = 1)
where (pf.from_desc in ( select from_desc from #from_desc ) or #select_form = 0)
and (case WHEN c.dist_version_id IS NULL THEN c.UCS_prod_no
ELSE
CASE
WHEN isnull(pt.version_control_ind, 'N') = 'Y' THEN xref.rss_prod_no
WHEN
isnull(pt.version_control_ind, 'N') = 'N' AND
xref.rss_prod_no IS NULL AND
xref.rss_film_id IS NULL THEN xref.rss_titleid
WHEN
isnull(pt.version_control_ind, 'N') = 'N' AND
xref.rss_prod_no IS NOT NULL AND
xref.rss_film_id IS NULL THEN substring(pc.glacct, 24, 5) -- ASR 3290687
WHEN isnull(pt.version_control_ind, 'N') = 'N' AND xref.rss_film_id IS NOT NULL THEN isnull(substring(pc.glacct, 24, 5), -- ASR 3290687
(
SELECT substring(dbo.ert_product.glacct, 24, 5) -- ASR 3290687
FROM dbo.ert_product
WHERE dbo.ert_product.product_id = a.parent_product_id
))
END
END in (select prod_no from #prod_no) or #prod_no = 0)