Fyi
It is about remuneration paid by their employers outside the R.O.C for services rendered in the R.O.C. during their stay (境內提供勞務境外雇主給付薪資).
https://www.contacttaiwan.tw/main/faq.aspx?faq=9
[Q9: How do foreigners, who stay in the R.O.C. for more than 90 days in the taxable year, file their income tax returns for income derived from abroad employers for service rendered in the R.O.C.?]
外僑綜合所得稅法規及實務.pdf - 財政部北區國稅局
pdf file if the link works
境內提供勞務境外雇主給付薪資
在台居留天數 境內提供勞務自境外雇主取得之勞務報酬
( 含) 90 天 不需計入課稅
91-182 天 按實際居留日數比例計算，按就源扣繳率 18% 申報納稅
(境外給付所得在台居留日數/所得期間)
183-299 天 按實際居留日數比例計算，扣除免稅額及扣除額結算申報
(境外給付所得在台居留日數/所得期間)
300 天 以上 1. 以全年度計課，不扣除年中短暫出境日數
2. 首度來台者，可扣除來台前日數
3. 當年度離台，可扣除離台後日數