der Posten
position
A job or position in an organization or company.
Er hat einen neuen Posten als Abteilungsleiter angenommen.
He accepted a new position as department head.

position / item / lot
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position
A job or position in an organization or company.
Er hat einen neuen Posten als Abteilungsleiter angenommen.
He accepted a new position as department head.

The meaning of 'Posten' depends strongly on context. In a professional setting, it means a job position; on an invoice, a single item; in trade, a quantity of goods; and in the military, a guard post.
Sie hat einen gut bezahlten Posten in einer internationalen Firma.
She has a well-paid position in an international company.
When 'Posten' means a job position, it is often used with verbs like 'annehmen' (to accept), 'bekleiden' (to hold), 'antreten' (to take up/start), or 'aufgeben' (to give up/resign from).
Er wird den neuen Posten als Geschäftsführer nächsten Monat antreten.
He will take up the new position as managing director next month.
Using 'Posten' for a job position is rather formal. It is often found in official contexts such as politics, administration, or large companies.
Für den Posten des Direktors gibt es mehrere qualifizierte Bewerber.
There are several qualified applicants for the position of director.
In the financial or administrative meaning, 'Posten' is often connected with words like 'Rechnung' (invoice/bill), 'Budget' (budget), 'Haushalt' (budget/household), or 'Liste' (list).
Wir müssen die einzelnen Posten im Budget noch einmal genau prüfen.
We need to examine the individual items in the budget carefully once more.
In a military or security context, there are fixed expressions like 'auf Posten sein' (to be on duty/at one's post) or 'seinen Posten beziehen' (to take up one's post), which means to stand guard or begin guarding.
Die Wache muss die ganze Nacht auf ihrem Posten bleiben.
The guard must remain at her post all night.
article
'der Artikel' and 'der Posten' can both mean something individual in a business context. 'Artikel' is an individual product that can be bought or sold. 'Posten' more often means a single item on a list or invoice, or a certain quantity of goods that is treated together. On an invoice, an 'Artikel' can appear as a 'Posten'. The words are easy to confuse because both occur in shopping, invoices, and inventory lists. If you are talking about a product in a shop, 'Artikel' is more natural; if you are talking about a line on a list or a quantity, 'Posten' fits better.

Auf der Rechnung steht ein Posten, den Lisa nicht versteht.
There is an item on the invoice that Lisa does not understand.
Der Verkäufer sucht den Artikel im Computer und nennt den Preis.
The salesperson looks up the item on the computer and gives the price.