In brief, "phase-in substances" are chemical compounds that are listed in the European Inventory of Existing Chemical Substances (EINECS). Substances in this inventory had already been placed on the European market before 18.09.1981, and were by definition exempt from notification under Directive 67/548/EEC (as previously for "new substances").
Furthermore, substances produced at least once in the EU between 01.06.1992 and 01.06 2007 but not placed on the market are considered as phase-in substances. These are mainly all substances within the scope of §16b of the Act on the Protection against Dangerous substances (Chemicals Act - ChemG).
Thirdly all substances which are considered as notified according to article 8 (1) of directive 67/548/EEC (< 1 to/a) but do not comply with the polymer definition (the so called "no-longer polymers") are considered as phase-in substances.
For the legal definition of phase-in substances please refer to Article 3 § 20 of the REACH regulation.
Non-phase-in substances are all substances which were placed on the European market after 18.09.1981 (i.e. the date of establishment of the EINECS for "existing" substances). They are either documented in the European List of Notified Chemical Substances (ELINCS), and have previously been notified under Directive 67/548/EEC or they had not been notified so far and therefore not placed on the EU market.
All substances which have previously neither been included in EINECS or ELINCS are by definition "new substances", which require registration prior to placement on the European market.
Annex VIX under REACH is a list of substances which are:
CMR - Carcinogens, mutagens and reproductive toxins
PBT - Persistent, Bioaccumulative and Toxic substances
vPvB - very Persistent and very Bioaccumulative substances