I'm sure you realised that netos is net something, but I think it's specifically this. Obviously "altas" are new policies and "bajas" are lapsed or cancelled policies in the period under consideration. "Netos" are the difference, which in the insurance business are referred to as "net increase" (in principle, of course, it could be negative if the bajas exceed the altas).

This is from a legal judgment in a case referring to the well-known health insurance company Sánitas:

"En el citado Plan se subraya expresamente que sustituye al precedente con efectos de 1-11-2011, la empresa sigue reservándose los objetivos, pero varían los pesos de los objetivos con respecto al precedente, ya que las ventas o captaciones reducen su participación al 50%, al igual que los denominados "netos" que, del mismo modo que la cartera, priman la diferencia entre altas y bajas."
http://www.poderjudicial.es/stfls/SALA DE PRENSA/NOTAS DE PR...

This refers to life insurance but it's the same principle:

"Net increase (Life Insurance) — The amount of new policies written and renewed less the amount of policies lapsed or cancelled. This calculation helps the insurer to determine the total amount of business the insurer has in force during a specific time frame."
The Complete Dictionary of Insurance Terms Explained Simply, 191
https://books.google.es/books?id=1ylxRpt35WgC&pg=PA191&lpg=P...

Same term and definition here, this time specifically in relation to medical insurance:

"Net Increase. :
The increase in the total amount of business an insurer has to force over a given period of time. It is figured as the total of new policies issued plus those renewed less policies lapsed and cancelled. (LI)"
http://www.medecus.com/glossary/term/n

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Note added at 3 hrs (2016-01-27 08:52:44 GMT)
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In other words, it means the net increase (or decrease) in the number of policies over the period.