They are not the same thing
The visa is issued by the Immigration Bureau and permits you to stay. The work permit is issued by the Department of Employment and permits you to work. Holding one without the other is a live problem: working without a permit is an offence for both the employee and the employer, and a permit is difficult to keep alive once the underlying stay lapses.
The usual sequence
In the standard case the Non-immigrant B visa comes first, obtained at a Thai embassy abroad on the strength of the employer’s documents. The work permit application follows in Thailand, and the one-year extension of stay is filed after the permit is in hand. Each step relies on the paperwork produced by the previous one, which is why the employer’s document set is assembled before anything is filed.
What the employer has to show
In the standard case the sponsor is expected to show a level of paid-up capital per foreign employee and a ratio of Thai employees per permit, with tax and social security filings in order. BOI promotion and some visa categories relax this. An application filed by a company that does not meet these tests does not merely fail — it costs the months it takes to fix the company first.
Where the two calendars have to meet
The permit and the extension of stay expire on their own dates, and each renewal needs the other to be current. Add the 90-day reporting obligation and the re-entry permit needed before any travel, and the practical answer is a single calendar covering all four — because losing one of them usually costs the others.
Changing employer mid-stream
The permit is tied to the employer and to the described role, so a change means a new application — and resigning affects the extension of stay that was based on the old job. The switch has to be sequenced so no gap opens in either permission, which is a planning exercise rather than a filing one.